All good points mentioned above. Let me add that in general, a zoom is only as fast as its "slowest" focal length and only as compact as its "largest" focal length. Wide angles (especially inexpensive ones) are often pretty slow and bulky, after all. The result is that on the kit lens, the widest (i.e. fastest) aperture at 55mm is f/5.6, compared to a 50mm prime lens, at f/1.4. The prime is 4 stops faster than the zoom at that focal length, meaning that a shot you could take at ISO 200 on the prime would have to be taken at ISO 3200 on the zoom (assuming both are taken at the widest aperture).
Even on a pro-level zoom like the DA* 16-50mm, the widest aperture is f/2.8, which is 2 stops slower than the prime. And the DA* is 5 times the price. (Yeah, yeah, that's kind of unfair. You'd also have to buy a $600 DA14 or $400 DA21 to fill in the wide end of the zoom, and maybe an $800 FA31 or $200 FA35 for the "normal" range.)
For size, speed, and (usually) image quality, it is primes all the way. For me, anyway.