imho they are great for some designs, not so great for others (in particular imho glocks, an accident waiting to happen) but in gen'l, i think they are a good idea, have a clip on my kel tec P32(which i carry all the time), and used to carry a 1911 all the time that had one, a bud had a S&W chiefs spec w/a clip/grip, and it worked great too. the only thing i can think of were they wouldnt be cool is glocks/steyrs/sigma design pistols, too much of a chance of a accidental discharge by the trigger hooking clothes, etc for me - but then again i "mex carry" my glock 33 all the time, loaded, just be carefull, ya should be anyway, but gen'ly i prefer to carry glocks in holsters if fully loaded, imho 1911's in particular are well suited for theses, firing pin safety/grip safety/safety lever seems like it would make them safe to carry this way, more so than a glock for sure
just my .02
greg