The one downside I've found with 40K, and Games Workshop in general, is they tend to make their brand spanking new released incredibly deadly so everyone wants to buy them. Then, they bring out "new" (and expensive) stuff for everyone else to level the playing field. Then, "new" stuff for the other race that was overpowered before. Rinse, and repeat!
I usually play Necrons, though have used Tyranids too. You can do a lot of damage quite cheaply (in both points and money).
Tau are a great option due to their ranged power. And yeah, they may be weaker in close combat, but that means nothing with die rolls - I can attest to that when my genestealer brood was wiped out in close combat with some firewarriors.