I run my WETTI M4 GBBR for CQB (indoors) and when the weather permits outdoors.
Otherwise - its an AEG.
If I had my choice - it would be GBBR all the time.
Guys are starting to look more into C02 mags for cold weather use...
As long as you maintain your GBBR correctly (ie. clean and lube it before and after use) you should have no problem.