I've had some opportunities to road test a few different services. Mainly iTunes, Video on AOL, and Hulu.
As far as iTunes go, it's a really good service for downloading the videos that you want. The download was quick and the iPod integration was awesome. Of course you have to pay ($1.99 when I tried it a few years ago), and it's all gummed up with DRM (so when I move to my Linux laptop ina few weeks, Battlestar Galactica won't be coming with me...).
Video on AOL gets a medium review from me. They have lots of content (which is a Good Thing) and most (all?) of it's free (another Good Thing). Unfortunately, the delivery system was so-so. I ran into a lot of glitches viewing content, things froze, I'd have to restart the show, the quality isn't great, and it lacks a true full screen mode. Also when I was using it about two months ago, there seemed to be only one or two advertisers and they only had one or two commercials each (I remember SC Johnson, Kohl's, and AT&T), so that got really repetitive. Still it's good for free, and again there's lots of content.
Hulu's awesome. It's The Answer, as far as I'm concerned. Since December I've watched more tv via Hulu than I have on TV. The quality of the video is excellent, I've had very few glitches when watching (especially impressive since it's in beta), the interface is elegant and intuitive, and it has just enough features to make it very usable (playlists, reviews, cast listings, etc) but not so many that it gets bogged down or overloaded. The sole drawback is that the content's just not there yet. It has plenty of shows--many of them old--but there may only be a few episodes for each show, and they disappear kinda quickly. None of this is a real problem, of course, since you're getting this for free. I'd love to see other networks license this technology or get on board with Hulu. After two months, I'd have to say that Hulu is my model for how I want to consume video content in the future.