I actually did a lot of time, even during day time games.
I mainly use 2 taclights:
1 High power for open areas;
2 Low power/ Strobe for indoor and breaching
Reason for 2 ligths, in the event of one failing for any reason, I still got a back up one. Also inside my stock, I got some battery tubes wich carry a full set of spares for each one of them.
For day time, it is a good way of signaling teammates from a distance without doing too much movement, wich proved really useful in many milsim ops I've attended so far.
The two lights I run are from Genesis, I run the Deron 750L (2011 model) and the Storm, both available at Sail Outdoor Superstore (In canada), for the rest of the world, maybe looking online you could get some good deal for them....The Deron being the most expensive at 200$ they are a really good and cheap compared to like Surefire and just as realiable IMO...
If you need more info, feel free to PM me and I'll do my best to help out.
Shadow out.