Buddy has one in a sub-compact glock .40 (not sure what the model number is). My opinions are:
1. It's weak. The dot is hard to see due to it's lack of power.
2. The blinking effect is supposed to make your eye find the (weak) dot easier. Honestly, it's just plain annoying.
3. The batteries. Special little watch batteries that cost about $15 to replace after only a few hours of intermittant use. My buddy's model uses 4 batteries (I think) at a time, and they run about $2-3 a piece plus tax. He says he can only find them at the jewelery counter at department stores/Target/Walmart etc.
4. The switch. Very easy to accidentally turn the unit on, especially when in a leather holster, and kill the batteries.
5. Cost. Find something better to buy, like a set of night sights and ammo.
Yes, this is 99% first hand knowledge, the rest I got from my buddy, and I take it as gospel.