Hulu is fantastic if you like the latest series from American broadcasters like NBC, ABC, FOX and quite a few others including basic cable channels (although no CBS or premium cable channels like Hulu or Showtime, though Showtime can be an additional plugin).
Think of it as if all the UK FTA catch-up services were available via one 3rd party service, and you'll get what it is all about, except it also offers movies. (If you love arthouse, their Criterion Collection is brilliant).
Quality is excellent, platform support is brilliant and I think the price is very fair.
I have the commercial version and have no issues with the number of ads for TV shows. Believe you me, there are a LOT fewer than on UK commercial TV, so if that is your reference, you won't feel inundated. (Generally speaking, each ad-break has between 1 and 3 ads, with my only gripe that they repeat the same ones).
While I don't mind ads in TV shows, especially the few that they have here, I don't like them in movies at all, so if you are also interested in their movies, go the ad-free version.