Went to the big one in Glasgow a few weeks ago - superb. Walked out with a bunch of CDs I couldn't get anywhere else, or at least, not for the money the guy wanted (£3 for a single disc, four for a tenner, £5 for doubles, four for £15, and so on). Many of the other dealers were asking daft prices for often readily available product.
I've been going to them for years, and they're declining to some extent with the advent of the internet, Amazon and people selling their own rather than offloading a bunch of discs to a buyer, wholesale, thus cutting out the middleman. I like them, but tend to be aware of what I want before I go in and then stick to that. I might buy the odd item I hadn't expected to find (found a three disc set of Springsteen's 75-85 set in Edinburgh a few weeks ago; first run edition, individually packaged with some slight damage to one of the jewel cases for a fiver - plays perfectly, sounds superb) and they're the real gems of course! Prices have plummeted as well with downloading, but others are still high. Watch out for some titles, which may be bootlegs. This is alright if you're looking for that kind of thing, the fairs are the best places to see the product upfront and some of the bootleg vinyl sets are well impressive, but prices often match and can easily go up to £100 or more for some items.
Ultimately, have fun, as they're a great way to see a bunch of guys of "a certain vintage" doing one of the few bits of shopping they like!