Best part of this place is the sheer amount of unique costumes. I mostly buy online because nearly all Halloween costume stores carry the same shtick for a lot of money and it all looks just as plastic as the packaging. But this place has things I haven't seen before, and I'm a costume junky.
Prices can be high but I'd say Black Cat doesn't have a lot of competition, not even in our crazy Vegas area. They carry elaborate costumes, wigs, gags, accessories, home decor and everything else. Everything is of really good quality, I love my wig I got for last year's Halloween costume. It was durable enough to take and hold product, and the hairs weren't sparse.
Just be sure you're ready to drop a little dough for a quality costume, but it beats the hell out of costume stores like Spirit. Definitely check it out!