I have 2 sets. One set were from one of those posh catalogues (reduced in the sale!!) and are great. The other set were really cheap from Asda and if I'm not very careful the cakes get stuck. I have almost perfected cake removal now though so maybe you just need to try them out a few more times.
I've just bought half a dozen packs of poundshop cases for Christmas presents - Maybe I should put a warning on them before I wrap!!