The M4 aren’t bas SSDs, but are slow compared to some others on the market. I think one of the fastest you can buy at the moment is an OCZ Agility 3, which incidentally my flat-mate has just put one in his 2010 MBP as well without any issues.
With the M4 you need to manually enable TRIM in OS X. Not too sure why, but I’ve written about this issue on my website when I upgraded. The article I wrote details that it was for Lion, but worked fine for me on Mountain Lion as well.
As a note, the article I wrote also talks about how not many SSDs would fit, but that issue is specific to my MacBook, and is not an issue for 2009 models onwards.
MrFart, how much faster to boot etc?
Well, mine boots in under a quarter of the time is took before, but I have quite a lot of backgrounds processes running, so you may or may not see the same effects. As I’d hoped though, with the SSD running, the CPU in the machine is now the bottleneck in performance, so I’m getting everything I can out of it now.
Hope all that helps.