It all depends on how serious you are about wanting a real relationship. Internet dating does work on both the casual side and the serious side...its important to *clearly* state your intentions in your profile on said sites. If you're looking for a LTR and possibly marriage, then SAY SO. If you're looking for something more relaxed, catching a movie and maybe some fun later in the night every couple weekends...say so! Also be prepared for disappointment. I dated one girl off of Yahoo! Personals for a couple months...how she acted online and whatnot was completely unlike what she was like in person. She knew that I wanted a serious ltr, and let me believe that she wanted one too when we talked via IM. But when we met in person, all she wanted was sex. Sure that is great news for most men, and yeah, it was fun for a couple dates...but ultimately it led to us ending the relationship simply because she didnt "hold up her end of the bargain". So be prepared for things like that.
The biggest problem with dating sites is the fact that in order to truely communicate you must pay for the service. Sure, you can browse profiles all you want, but if you want to actually talk to someone, you'd better be ready to whip out your credit card. Also beware of their "get 6 months free" schemes too. They usually have fine print attached to them.
As for the difference between sites themselves...again it comes down to what type of relationship you're looking for. Take True.com for instance:
Yah...seriously.
Certain sites are geared more for actually finding love than others. From what I can tell, Match.com & chemistry.com are geared for finding a relationship compared to fling & true. Yahoo! Personals is also a decent site...but like i said...you have to state what you want since theres some serious people and some "out for a hookup" people on all of these sites.