I use Elixir, not because they sound particularly better than cheaper strings, but because they last 4, maybe 5 times as long. They've also got a really smooth feel due to the coating.
Generally, aside from absolute dirt cheap sets which the majority I've found to be clanky and rough, I've found most £12 to £30 sets are much of a muchness. Roto RB's and Fenders for example are to me as good as Slinky's, D'addario etc, but can be picked up for nearly half the price. Pricey strings that did sound particularly good were DR and Dunlop, but again they didn't really last long enough to justify the cost unlike Elixirs.
I guess even if I could get 3 or more decent sets for the price of one Elixir set, its still 3 times more string changing and breaking in etc. So yeah, probably one of the most expensive sets is my preferred choice, but apart from them, I'm not sure you always do 'get what you pay for'.