I've been building models since the late 60s, and I've never used an airbrush. I use spray cans for basecoats and (depending on the look I'm trying to achieve) hand painting, dry brushing, and/or washes for detail and weathering.
If you're trying to get a smoother finish from a spray can, heat the can before you use it. I soak mine in a bucket of hot tap water for 5-10 minutes (I put them in a plastic bag first so the water won't damage the label or rust the can) and shake them thoroughly before use; the heat thins the paint slightly, which results in a nicer finish. Nice thin coats, and let each coat dry thoroughly before applying the next.
That said, for natural-looking effects like shading and highlighting IMO you almost have to use an airbrush (unless you're a really skilled brush painter). Also, airbrushes are invaluable if you're constantly mixing custom colors that have to be sprayed onto large areas. I'm sure I'll get one some day, I just haven't gotten around to it yet.