Here's my experience from some solid state drives when used on a 2.4ghz Unibody 15" MBP:
240gb OWC Mercury Extreme Pro SSD 3G: Ran fast, but noticed LOTS hesitation with apps when opening. I cost me $400, and honestly, it wasn't worth it at all. I returned it.
120gb OWC Mercury Extreme Pro SSD 3G: Ran fast, had no noticeable side affects, and I picked it up pretty cheap...~$130 if I remember. Worth the price.
60gb OWC Mercury Extreme Pro SSD 3G: Did not perform as fast as the other two OWC drives, but ran consistently all the time. Bought it for just shy of 80 after discounts, which made it an awesome deal.
30gb(32gb real space) OCX Agility Series SSD: Ran well. Was my first ssd on my MBP. Ran very consistently all the time.
With all those, I find myself with a 500gb WD Black. Trusty, reliable, and CHEAP!
SSD prices are still to high to justify the few gained seconds to an average person. Save your pretty penny...for now.
With that said, if your mind is made up on getting one, you have to know what SATA version your computer has. If your computer has SATAII, most SATAIII drives will scale down to it, but issues have risen with compatibility problems with MBP's. If your computer has SATAIII, then getting an SATAIII ssd would be the obvious choice, unless price is an issue. My MBP is limited to SATAII. Almost all new Apple computers use SATAIII now.
If you've got money to blow, go for it. Just to make sure though, don't get anything that doesn't cross the 500mb/s border.
For me, I'm not going to worry about an SSD until I upgrade from my trusty MBP