Okay, so after experimenting with a lot of different (free) game engines over the past several weeks, I thought I would put down my thoughts on four of the most popular ones for everyone to see. Many if not most of the people reading this will have used one or more of these; feel free to add your thoughts in the comment line. Please understand that I write this as someone who is a novice in all these programs, specifically for other novices who want to compare and contrast their options without jumping into anything. Enjoy.
The Good: SP is very simple, on one level. Basic collision stuff and what-not is fast to add, and can be quite amusing to experiment with. It is also nice if the user has experience with Sketchup (why else would you be using SkethcyPhysics anyway??), because the user doesn't have to learn a new interface (scaling, rotation, viewing ports, etc). The mesh can also be edited directly within the program, which is a very nice feature if you are prone to forgetfulness (like myself).
The Bad: SP is very poorly documented. Sure, there are a lot of tutorials on Youtube, and a few websites with useful info, but there is no in-program help, and much of the online help is outdated. Another issue is that SP runs slightly different on every computer, so what worked for Joe Shmoe on Youtube may not work for you. A lot of features are also rather finicky, especially where joints are concerned, and it's easy to screw everything up if you make a simple mistake. Also, leave your code at home: it won't help you here.
The Ugly: SP is extremely limited where speed is concerned. A less advanced computer can have problems with just a few thousand P-gons, and if your machine doesn't have OpenGL capabilities, you can forget all your neat little textures.
The Bottom Line: SP is a lot of fun for goofing around, but if you want a dedicated game engine, this isn't for you.
The Good: Blender is a multilevel package. It is not merely a game engine, but a quite advanced modeling, rendering, and animation suite as well. This is very nice because you can simply pause your game and edit your mesh within blender if anything goes wrong. Blender is python-based, so a little bit of coding experience will go a long way. Also, there are literally thousands of online resources for Blender, including several very dedicated forums. If you don't know how to do something in Blender, you can google it, and recieve half a dozen useful answers. As for the tool set, Blender's extensiveness is beyond compare (Not to even mention the thousands of plugins available). I will not go into details for the sake of anyone technical-vernacularly inexperienced, but look at it this way: If SU was connect-the-dots (you HAVE played connect the dots before, right?), Blender would be Photoshop.
The Bad: Blender is updated ALL THE TIME, at least, compared to other programs like SU, Unity, 3Drad, etc. Basically, by the time you get used to a version of Blender, it's outdated. Sorry, but that is the cold, hard, python-powered truth. But don't worry, they only completely overhaul the program every few years or so. And if not being on the constant edge of technology doesn't concern you, this isn't a huge problem. On another note: unless you take the time to get a deep understanding of how Blender works, you will be constantly frustrated by problems that could have been fixed with the click of a button had you known about that feature earlier.
The Ugly: Blender is complex beyond your wildest dreams. Don't even think about making a game in blender unless you have a lot of time and energy on your hands, including hours to spend pouring over tutorials. Also, like SP, if your machine doesn't like OpenGL, your Blender gaming days will be few.
The Bottom Line: Blender is a lot of work to get into, but if you have the time, energy, patience, and right computer, there are many rewards to be had.
The Good: The word for 3DRad is simple. Unlike Blender, all that Rad does is build games. This means that the interface is simple, and your engine won't be cluttered by hundreds of features that you'll never need. The built-in help topics are simply spectacular, ranging from adjusting the shocks on your car to basic coding principles (Coding can help quite a bit in Rad). The actual construction interface is slightly odd, in that it is more or less "Drag and Drop" based (even where coding is concerned!). The program comes with a variety of prebuilt/coded assets that can be compiled in minutes to make a simple racing game. Racing is really where Rad Shines. Included are pre-coded physics for everything from an Enzo to a Smart Car, and the interface makes modification easy as "Input a new number into the empty box. hit enter." Graphics-wise Rad does great on virtually any system, and only slows somewhat on older machines when a lot of p-gons are involved (a lot as in upwards of 20,000). Because you have to import mesh from other programs, there are plugins both for Blender and Sketchup to support conversion, making the creating of new assets a quick and easy process.
The Bad: Like Blender, the best thing about Rad is also a bit of a problem. The simplicity of Rad's drag and drop system CAN make the creating of new assets rather tedious, importing one file at a time and converting it into a format that Rad can use. There is also very little online support when compared to more popular engines. It is also designed to work specifically with NiVidea Graphics cards: be ye forwarned. Also be forwarned that you will need to learn how to use a 3d modeling program if you want to create custom assets.
The Ugly: While Rad is excellent in Racing games, it falls short in many other areas. A user would be hard pressed to make something more along the line of, say, and FPS, because the engine itself is so simple it doesn't handle complex animations. Finally, it is nearly impossible to add new features to a prexhisting object without plenty of nasty coding.
The Bottom Line: 3DRad is an insane ammount of fun, and can be mastered in little time. Be prepared to get bored quickly.
The Good: Unity is the "Rolls Royce," "Macintosh," or "Fischertechnic," of game engines. It does exactly what it is supposed to, and it does it nearly flawlessly. Unity can best be described by it's name: It is slick, quick, and trim, without a truckload of options staring you in the face. It is wicked fast on virtually any system, turning out hundreds of thousands of P-Gons at ten times the rate of comperable programs. This program is specifically designed to work with any graphics system. The free version includes game exporters for Web applications, Macintosh, Windows, and Adobe Flash, with more addons availible. It can inport virtually any asset (Collada, .blend, .obj, .3ds, .x, you name it, it does it) without installing plugins (roughly five times Blender's extensive list of import/export). The free version is virtually the same as the professional version, mcuh like Sketchup, granting you mainly proffesional level tools for no cost. The basic interface is simple and straightforward, and the help topics are the most extensive I have seen (there is a help button next to every window or tool). With some coding tricks up you sleeve, virtually anything is possible with Unity.
The Bad: Unity has some finicky features where setting controls are concerned (I have yet to figure them out). It also stores and retrieves your projects not as files, but as folders. Thus when you load your project "File," you are actually loading a folder. In practice this is very efficient, but it takes some getting used to. Another thing to consider is that Unity is not standalone: the user will also need to become familiar with a 3d Modeling program to create assests for his game.
The Ugly: It is very difficult to do much cool stuf in Unity without some coding abilities. Nothing fancy, but at least a basic understanding is required. Unity is also very heavy where jargon is concerned, so be prepared to spend a lot of time in the help section figuring out what stuf flike "Linear X occluding drag force" means. Unity will also take a lot of time to learn, you can't just pick it up and use it to build a game in an afternoon.
The Bottom Line: Unity is for serious game developers. It outshines every other engine's capabilities, but it will take a lot of work to get into.