I own a 2009 Macbook Pro, a 5,5 MB991LL/A to be precise. It was the first Mac I ever owned and still my favorite. When my iMac started to act up I decided I needed to upgrade or buy something new to use as a primary. In the end, I decided to take my Macbook Pro from 4GB of RAM to 8 and add a 256 GB SSD.
I couldn't be happier with the outcome.
If you work with documents and projects of a size that require that you watch a loading bar a dozen or more times throughout your workday, a Solid State Drive is a good upgrade. xBench puts my MacBook Pro at 25 points higher than the average Mac Pro. Put into the proper context, my two-year-old Macbook Pro is a madman when it comes to most of what I do.
Aside from the performance boost, there were a couple of other bonuses I wasn't really expecting. First, my Macbook Pro is very quiet compared to before. It makes almost no sound whatsoever unless I'm playing a game and the fan kicks in. Second, it gets slightly better battery life now. Not a huge amount, maybe 25-30 minutes of additional operational duration.
If you are trying to decide between upgrading or buying a new machine, I'd take a look at an SSD as a possible option. Depending on your computer, it might be a significant performance boost for less money than a whole new machine. The difference with regard to my MacBook Pro was like night and day.
There's no Trim Support for the SSD I chose, but hopefully the release of the Lion version of Mac OS X will change that.