Like drpepper said, you can decompile, but some of the brush entities (from my recollection, areaportals and some triggers) will be unassigned. Despite the stigma surrounding decompiling maps, it's perfectly okay to decompile a map if you wish to see how a mapper did something or if you want to learn how to detail a map/make good prop placement. As long as you don't attempt to alter it and release it as your own, you're okay.