The trick is to pop in often, as they change stock literally by the week. So you might find nothing but then the next week they'll have some good stuff in. If there is something nice the 'average' sizes tend to go out of stock fast so they're not the type of purchases to sleep on.
Best recent purchase has been a 2m dual headed (iPhone & mini USB) charger cable for £3.50. Works perfectly and I can now charge my phone and kindle (separately) off the same charger and the length is excellent. You'd pay 6x that price elsewhere.
I've also got a fabulous sequinned clutch bag I got reduced for £2 about 6 years ago. It looks so so expensive, is heavy and satin lined with a removable strap, you'd never peg it as primark. I thought I'd lost it during a sort out recently was genuinely gutted, but it turned up in the bottom of a box a few months later I was overjoyed!
Good for kids clothes and holiday wardrobe. Also for 'fashion' items like currently e.g. Bardot tops and the frills trend that you can buy cheaply to update a look but not break the bank and don't feel guilty about charity shopping them after a short while.
My nearest primark is over 4 floors and can be hellish, you have to pick your time. Never go in with a pram If you can help it! You'll be sweating and need a stiff drink. I used to go at 6pm after work when I worked in town which was much better than trying to go at lunchtime or the weekend.