good for sunning herps (heat, natural UV), crap for everything else.
make sure you keep water in there if using for sunning herps, as the black mesh seems to attract alot of heat, the temps get quite high, i generally position the enclosure half in the sun, half in the shade created by the house, but keep an eye on it though, as the shadow will obviously move.
I pretty much agree with Azztech but they can also be handy to use as a temporary holding cage for while you clean out an enclosure.
Also as Azztech said they do tend to attract a fair bit of heat so they aren't something I would recommend putting your snake in outside and then leaving unattended for a long period of time. I used to use mine to let my snakes have a sunbake while I was out doing the gardening or something like that so I could keep a close eye that it didn't get too hot.
One thing to be warned about though is that these are no different to a standard enclosure... some are well made, some are not! The first one I bought was (i believe) one made by the pet shop owner and it had some pretty dodgy stitching and didn't last very long. The next one I bought was an exo-terra one and it was brilliant even though it cost a bit more, so if your looking at bvuying one make sure that it is well made just like you would a normal enclosure, it will save you having to replace it in the long run.
Yeah, gotta agree with both of the above posts. I use mine for sunning (half in shade, as Azztech says, although watch out for moving sun!) and for temporary holding when cleaning out the cage. Very useful item (Thanks Afro!)