I have one of the "new version" marui ones...very fun to play with...totally gives ur position away though! I like using them esp to adjust hop up (easier for me to follow my shots...i always have difficulty doing that...bad eyes!)
Its good as long as you can mount it! (I have it on my SR16...my g36c couldn't 't use it unless i get an extension and my SCAR has the opposite thread so I needed an adapter for that one too hah).
If you get one that uses AAA batteries...pull the batteries out or remember to turn it off...the units are hogs when it comes to energy consumption.