I recently purchased a PS3 120 gig, which is the first console I've owned. Before that I played primarily on the computer (though I have played a few games on other people's systems, most notably Forza 1, Okami, and Shadows of the Colossus). With that said I'll weigh in with my opinion.
A big advantage of the ps3 is the ease of playing games. This is much more noticeable to me since I use Ubuntu as my primary OS. That means if I want to play a game it must have a linux version (few do), try and get it to work with WINE (hit and miss), or reboot into windows (probably best solution but I almost never do). With the ps3 all I have to do is turn it on, switch the input on my monitor, and insert the disc (or select game if on PSN). I now game around 2-3 hours a week. When I had just my computer I would rarely game, and when I did it would occur in "binge" sessions.
Another advantage is that I don't have to worry about capable hardware. When considering purchasing Bad Company 2, I wondered if my PC would be capable. While I understand that new games can be played on relatively old hardware I fear that I'm lagging a bit too far behind. For what it's worth, I am still on s939. Conversely, if I were to purchase it for the ps3 I am practically assured it will run smoothly.
Regarding arguments about costs for hardware and accessories and whatnot...
As others have mentioned, the cost of a computer isn't that great. My core system was bought used (athlon 4400, 2 gig ddr, 8800gt) for $205. I got my case, ps, and optical drive for ~$70. Until recently I had been using two old hard drives, but even upgrading those didn't cost much, given today's sales and hard drive prices. These costs have been spread out over a roughly 5 year period (going by the oldest component). Sure, if you need to have the latest and greatest a computer will outcost a console, but if you're content with older technology a pc can be as cheap as a console. Plus, the ability to upgrade and do repairs yourself is quite handy.
I agree that counting the cost of a display for consoles is stupid. Computers need an output as well, and you could use whatever size you want for either input. However, while tv's do more than just display console games, a computer does *much* more than display pc games. The utility of a computer far outweighs the utility of a television.
In my case, the pc has the advantage in game costs. There are a ton of older games available for dirt cheap and new games tend to fall in price relatively quickly. There is less price elasticity with ps3 games in my experience. LittleBigPlanet still seems to be around $30 used, while the GOTY version is pushing 45. Demon's Souls has a similar price point and unfortunately there doesn't seem to be a demo. I would consider renting games, but at my anemic pace I would be lucky to finish one game a month. And while console games don't have to deal with DRM, I avoid that issue on the pc by not buying games with DRM.
I have no regrets about purchasing the ps3, and I absolutely *love* flower. It's a nice way to spend 15 minutes winding down at the end of the day. Mirror's Edge is pretty cool, although I would prefer to run it from the hd; the disc accessing sound the ps3 makes is very grating. I have played the demos for LBP and Valkyria Chronicles and am waiting to find them at the right price. I also tried Brutal Legend, which has a delightful tone, but the gameplay itself isn't up my alley. It's a game I would rather watch someone else play.
As someone else said, if you can afford it get both a computer and console and enjoy the best of both worlds.