I've used both the M4 and the T-1/H-1.
I prefer the T-1 or H-1 due it's weight, function and capabilities. The M4 seemed overly bulky to me but it is a well designed and solid sight that I would use if I didn't have an H-1. (I got the H-1 because I'll probably never use NVG's again - thanks Abhobbyist! - he had a spare H-1-). The M4 looks cool if you're going for CDI factor.... the T-1 or H-1 ..due to its size shows that you are secure in your masculinity with a small sight. Small sight must mean that you have big cajones!
I borrowed foxbat's Aimpoint magnifier during his course and found it useless for me until beyond 50 yards but for those needing longer range precision, it will fit the bill out to whatever distance the shooter is capable of engaging to.
So, if you are accurate with just the T-1 out to 100m, you should be able to effectively use your T-1 and magnifier out to 250 or 350m or even beyond.
I would use it if sitting on containment then would tip it off if an entry arose.
Unlike Wicked, I prefer the Larue tip off mount rather than the twist off mount so that I could get the magnifier into play quicker however, it would be mission dependent. He is absolutely right about it hanging up if rappeling, entering etc... if I was doing an entry into a normal house, business or school, I would take the mount off completely. I'd run it in the great outdoors though. (that being said, unless I return to LE, I won't be doing entries anytime soon except on courses!)
Oh, and One Shot also carries Aimpoint gear.
Practise is your friend!