In reply to Oli:
So far, I have tried the following...
Minitraxion
I hated this and it also failed on me on my first session with it causing it to be declared unusable by the UK distributors (whom I returned it to). To be fair, it did lock very well and travel very smoothly but the teeth on the actuating cam are vicious and you could see on a rope afterwards where it had gripped and penetrated the sheath. The reason it failed was that I used a standard krab so my fault I guess. Still didn't like it though.
Reverso in autoblock mode
Like this but you have to pull rope through very firmly and rerig for abseiling down.
Shunt
Good device, but personally didn't get on with it. Also should be read up on and researched fully beforehand as there are issues you would be well advised to be aware of before trying it.
Grigri (non-modified)
Bought this very recently and have only used it a couple of times but really like it. As it is unmodified I have to pull rope through, but this is MUCH smoother than the Reverso, and I find pulling the rope through is good practise for finding positions to clip from! It is strong enough to take a fall onto backup knots in the event of some sort of failure (unlike the shunt), and I put a lot of trust in its ability to hold a fall having played quite a lot with jumping onto it from various heights / angles.
I may modify this at some stage and if so will happily report back my findings.