If you mean Magpuls / Magpul magazine assists I'd say they are useful. Depends on your budget though because say you have five magazines - each Magpul is going to be around £3 each...that's £15 :|
I dont bother with magpuls personally, much easier to wrap black/camo tape around thin card and then tape it to the bottom of the mag. D.I.Y. for the win!
I think it honestly looks pretty sh*t if you do that - or at least my attempts did anyway.
It works though so it's worth doing! Magpuls are expensive (like everything over here) maybe worth it in the US, but for £3 I'd say no. At least look to buy some off eBay.