Before I met my current girlfriend, I was on plentyoffish.com for several months. It's an interesting site because your experience is very different based on what your settings for. People whose settings you don't fit and who don't fit your settings are automatically excluded.
Once you say one thing or another with one account, it keeps things that way and you can't go back. For example, messaging people with certain interests forever marks your account as having messaged people with those interests.
This system seemed to either really work or really not work for my friends who I've spoken to about the site. For me, the site just didn't work. I've met and dated people successfully both long and short term on many different dating sites--okcupid, craigslist, match, and salon most notably. In my time on plenty of fish, however, I had perhaps two first dates, neither of which worked out.
One advantage plentyoffish.com has is that it's free. However, okcupid and craigslist are also free and have both been more successful for me in my times dating online. In fact, I met my current girlfriend on okcupid.
If you enjoy browsing around and flirty with people a bit online here and there, I can't knock plentyoffish. If you're serious about meeting someone--for just about any reason short or long term--I'd look elsewhere.