The clothes tend to be the right side of 'fashionable', ie the cuts are modern and flattering, the price is very comparable with and often better than the other UK high street rivals, and most importantly the quality for the most part far surpasses everything else in the price bracket. It's (generally) far better than buying from Zara or H&M, which feels very 'fast' in comparison.
The GAP comparison is probably apt, although one thing I really appreciate about Uniqlo (especially compared to GAP) is the lack of branding and logos. You can easily build a complete wardrobe (work/pub/play) - especially if you're buying base pieces. Which is probably the reason they get a lot of love on /r/malefashionadvice/.