There have been many reports of players using the Myth/Myth 2 regs that leaked. I myself have owned 3 in the past of which 2 have leaked and still own 1 which has had no problems.
The problem lies with the small oring on the piston of the reg. Some were fine, some looked like they were either smaller or worn down.
I bought these from LIPs, and with there awsome customer service and aftercare, they sent me out replacement parts, which was a new piston with the oring fitted, they even sent out a small amount of air-smith grease(very thick stuff) in a small sealed bag to apply on the oring.
Never had a problem with them since. Several team mates have been using Myth 2's without problems for over 12 months now, so the issue may have been retified by now.
If your still not sure, then give LIPs a call, im almost certain they can do a Geurrilla bottle and the new Ninja reg combo.