The CompM4/M4S is a solid scope, and it's built like a friggin' tank. It weighs about the same too. I have an M4S and I love it. It's a great optic, but it's an expensive scope. When I look at the cost of the PRO, I'm not convinced that the M4/M4S is worth the added expense for what we are doing on duty.
Our squad rifles have PROs on them, and we're at 2.5 years now with them and never had a single problem with any of them. I'm not a fan of the height, as the PRO is essentially an absolute co-witness (just an RCH off, but close enough). I have the M4S and T1, and my next Aimpoint will most likely be a PRO transplanted into an ADM cantilever Lower-1/3 Co-witness mount. As much as I like the light weight and compact size of the Micro, the small tube size of the optic makes for a much more narrow eyebox when you're behind the rifle. When working in alternative firing positions like rollover, urban prone, SBU etc, I have a much easier time acquiring the dot with the M4 or PRO than I do the T1.