+1 on the link above.
You'll get a lot of opinions on this. And every one of them is coming form musicians who are qualified to weigh in on the topic. Here is mine:
Personally, I wont play without one.
In my opinion, you dont need to sell your car, so that you can afford a botique compressor. If you're willing to drop about $100, you can get a good compressor.
But I dont reccomend getting the cheapest you can find. If you do, you'll find yourself battling all of the issues that compressor can cause in worst case scenarios, such as noise, breathing/pumping, "glassy" high end, etc. A better compressor will help battle that.
Aphex makes a great optical compressor called the Punch Factory. Great for compressor newbies, because it only has two knobs: Gain and Volume.
http://guitars.musiciansfriend.com/p...ELAID=26018247
Then once you get more comfy and learn more, you may find yourself wanting more control and seeking out a three or four knob compressor.
Here's the compressor that I use, and it only costs $70. I love it. It's quiet, it's sturdy, and it allows me the extra control that I was missing when I used the Punch Factory:
http://bass-guitars.musiciansfriend....dal?sku=150826
Go to GC and play with the compressors there. You'll hear the difference between a compressed tone and a non-compressed tone. Then you can decide for yourself which is better suited to your tastes and style.
Compressionn...? 
