I'm a fan of trying out cheap solutions and posting the results.
I recently bought a $10 Solarforce ebay clickie tailcap for a G2, as I've got several G2s and a 6P with twisty tailcaps that aren't optimal for weapon use. I figured it was worth a try, since the SF tailcaps are $30-40.
It arrived a few days ago from Hong Kong (long shipping time, as usual), and it doesn't work at all on any of my G2s. It works on my 6P, and my guess is they sent me the wrong one, though generally 6P and G2 tailcaps should be interchangeable. I believe the problem is that the tailcap isn't making contact with the G2's metal sleeve insert, while a 6P has good contact as soon as you screw it on, due to the metal body.
Anyway, it's a good, solid click action - just a little grittier than a SF tailcap, but not bad. The big problem is that it's a reverse clickie - a soft press doesn't turn it on until you've clicked it on. Then, soft press turns it on and off. This is the opposite of what you want from a tac tailcap, of course, and they didn't mention it in the ebay ad (and I didn't ask).
So, as always, there's a reason the cheap stuff is cheap. It might work for you, it might not. With CAA gear, I've had good luck with some of it, and not so good with others. It might be worth a try for a switch, but you'd want to test it pretty thoroughly before committing it to defensive use.