Got one a couple of weeks ago. I have only had a play so far, but am doing a nav. assessment this weekend which includes some night nav. on Dartmoor, so I'll have more to say then.
First impressions are good - it is seriously bright. My son has got a LED Maglite, which at 77 lumens is pretty similar to the Gamma. It is way brighter than that. The dimmer function works well (though I think it will still be too bright for use reading in a tent), and zooming from spot to flood is easy & works well. The main head unit seems very well made. I am slightly concerned about the battery compartment which feels cheap and shoddy, (though easy to take the rubbery back off - too easy?), but like I say as I haven't used it in anger can't yet say if it will turn out to be a problem.
I will say though that if Alpkit had had them in stock I would probably have gone for a Gamma - the money I'd saved would have gone a long way towards paying for a new sleeping mat as well. What are you using it for? In what way is the Gamma not meeting your needs?
Petzl NAO does look nice, but I can't go anywhere near justifying the price for my needs.