We had our Sureflap installed last week, and so far I would definitely say it's worth it despite drilling a blardy great hole through our back wall. Archcat has had to defend his garden already against one particularly persistent neighbourhood cat, and I feel it would be psychologically disastrous if that cat managed to enter his 'safe space' inside.
The other good feature is a four-way door (I think other flaps have this too), so in the evening for instance we can set it to 'entry only but no exit' and then keep him in for the night.
Archcat uses the Sureflap, but he prefers to use the back door if he can persuade one of us to open it or can slip out while it's open anyway. I suspect he's nervous about the flap being in a wall because he can't check in advance that the coast is clear when he comes out of the tunnel. A flap in a window would work better in that respect.
"Also I recently watched a TV programme where the resident cat ahd a microchip cat flap, but the neighbour's cat still managed to get in as he discovered he could hook the flap door inwards (ie the wrong way not cip activated) and sneak through!" I watched that too, just before we were going to have the flap put in, and it worried me enormously. The programme is 'Cat Wars' on the BBC. The invader cat's owners were arseholes, TBH, effectively laughing at the other cat's distress. In that case I think the invader was particularly ingenious and determined, combined with a shy resident cat who suffered from extreme anxiety and was in no position to defend his territory. You have to know what your cat's personality is like before deciding to go through with it.
YY to cozie on shopping around. We're not in the UK, but got our kit from the local equivalent of bitiba and it was way under RRP. Have just looked and it's 60 quid, but you need the adapter too which is 9 quid. And if you want to put it through the wall, like we did, you need quite expensive tunnel extenders. All in all, the equipment plus installation cost about 200 quid.