I agree with Jason. There are good kits out there, especially ones meant to cover you for an individual hatch. If you're starting from scratch, not a bad idea at all. Like now, if I didn't have any BWO's of any type, I'd consider a BWO kit, with nymphs, emergers, and duns of the same insect.
However, most kits claim to give you everything you need as a trout fishermen, 2 of this, 2 of that. Not a good idea. You end up with 2 or three patterns you'll actually use and a bunch of stuff you'll never use. And of course of that pattern you will use, you lose both flies in an hour and are just left with the crap. I still have salmon flies in my box from buying one of those kits. Unless I make a trip to Montana, I don't foresee ever taking them out of the box.
Here's the secret. Based on stream and time of year, you can predict what flies you'll use to a 95% accuracy. Get a few generics, like buggers, egg flies, greenweenie, walts worm, scud, etc. But other than that, if you're going to stream x in month y, look at a hatch chart, decide what flies you'll need, and buy a half dozen each of that nymph, dry, and emerger. Whatever you don't use is a start for next year, you'll be back. Then the next stream or time of year comes along, and you get a collection of a new bug. After a season, you have a collection of all the bugs you normally fish, and all you gotta do is replace lost flies.