I just installed 840 PRO to my 2 and 1/2 year-old DELL XPS L502X (Core email@example.comGHz, 6GB) and it is FAST. It used to take 1 minute and 22 sec to get to the login screen, but now its 19 sec.
Data Migration tool failed once, toward the end because DropBox recognized a new drive and pupup a dialog to save pictures on it. I closed all programs like evernote, google talk, etc. The second time it worked. Once I get the data transferred, then it is just get one screw driver and replace the HDD with SSD.
I basically extended the life of my notebook by maybe a couple of years? (and I still have 6GB=>16GB memory upgrade)