I personally own Dominion, Seaside, and Prosperity.
I enjoy Seaside the most out of them all, and advise any one with just the base game to pick this up as their first expansion for these reasons:
1. Balanced cards. There aren't many cards in this set that are way to swingy. Sure Ambassador and Sea Hag are, but they aren't as bad as Tournament or King's Court.
2. Interesting Cards. There are some really cool cards, functionally, in this set. Ambassador is a nice twist on trashing. Lookout, though weak, is still one of my favorite cards to play with. Not to mention all the Durations! They are really fun to play with (as long as you remember to leave them out!).
The Base Dominion is of course good. But the cards there are pretty bland, but that is expected for the first set.
Prosperity, in my mind, has way to overpowered of cards, and can have most games with it comprised of you just sitting there while your opponent goes, "City, City, City, King's Court, City, Goons, Goons, Goons, buy 2 Goons and 50 thousand Coppers, revealing a Watchtower to trash them,". I know some people really enjoy these sort of games, but I don't.
Overall, I'm pretty content with what I have. I'll probably buy Dark Ages just because it's a blast (been cycling through cards by using blanks
). We have a pretty big gaming group, and Dominion with 7 players doesn't have much strategy. Will most likely buy a couple sets on Goko (once they get everything figured out) and play 1 on 1 with my friends there.