If you like your beer tasteless and uncomplicated, this is not the place for you. If you like a bit more nuance and flavour when it comes to your brew, then come on down! Dovetail is a small-ish Belgian restaurant and bar, although to my mind the emphasis is more on the bar front. They have a range of Belgian (and some Dutch) beers on tap, as well as a pretty good sized selection of bottled beers as well - which is essential to a good collection as a lot of the best Belgian beers (especially Trappist ones) don't come in ye olde keg.
As I noted, the place is a bit small, and it was quite crowded with City types the night I visited. Still, my drinking partner and myself managed to find room to sit and enjoy a variety of delicious beers. There are a bunch of old beer posters and signs adorning the walls, and a big ol chalkboard to promote the food specials of the day. We didn't have anything to eat apart from some peanuts, so I can't really comment on the food. Nobody else seemed to be eating either, however, but plenty of calories were consume via liquid refreshments.
There are better Belgian bars in London, but this is, to my mind, a pretty good one. It is probably a great place for a mid-afternoon drink, as it would likely be less crowded and more mellow at that time, but it's also good for a (small) group of friends to gather after work and have a few. Don't be afraid to try new stuff, and ask advice in terms of what kind of flavours you are looking for, you can find everything from spicy hops to fruity tastes and coffee or chocolate notes. Nice.