I realize this thread is a little old but it didn't sound like JackedinMI got his original question answered as well as he'd like. I'm currently running 500mg of test enth weekly with 100mg of mast eod. As an out of season body builder (in other words, a little bit higher body fat than I'd like, just to bulk) at about 15% bodyfat, I was hesitant about the mast just like you were just because I had heard that it sometimes was found to have more androgenic side effects over 300 mg per week, but that it would harden you up. Skipping all the jargon, it HAS kept me harder than I normally feel at 15% bf, and it's decreased the amount of water that I store. Decreasing water storage means that you could be sacrificing a little bit of strength, but the affectiveness of the test is also increased, so it's a nice balance. Pros: I feel harder, and less soft than I would on just test. It decreases stored water which contributes to that cut look. It has little to no androgenic side effects at 100mg eod IMO. It increases the effects of the test. ie strength, repair time, endurance, craziness, huge biceps, etc. you get the picture. Cons: Pinning oed and sometimes every day just because of how my schedule falls SUCKS. MAKE SURE you rotate your injection sites. It's very important that you're comfortable with pinning at least glutes and quads so you can rotate left quad, right quad, left glute, right glute, repeat, or you risk turning a site septic and/or developing scar tissue which just means it hurts like hell to pin there anymore. Another con, the chance that you'll have decreased strength due to less water being stored. IMO, im ok with that, and i also didn't experience that. But I sometimes hear that's the case.
In conclusion: I recommend it to all levels of experienced/non-experienced users as a supplement to whatever you're stacking. Just be comfortable rotating injection sites.
Hope that helps.