I build what I call "technical LEGOŽ" (any suitable combination of Technic, Power Functions, and Mindstorms, studless or otherwise) almost exclusively. The reason is simple: I love gadgets -- especially mechanical ones -- and the science and technology behind them. Technical LEGOŽ brings out my inner inventor/designer/engineer in a way that no other medium I've tried can match. I'm generally more committed to functional than aesthetic goals but go for both whenever possible, sometimes with a dash of whimsy thrown in.
Viewer discretion advised: If you get squeamish at the mere thought of modifying a LEGOŽ part or indignant at the sight of a non-LEGOŽ component, you may want to cover your eyes here and there. I try hard to be good (that's most of the fun) and usually succeed. But if LEGOŽ doesn't make something I really, really need, and there's just no acceptable work-around, I'm not above modifying parts or even (gasp) resorting to non-LEGOŽ items to fill the void. In particular, I've found that stooping to the use of parts of parts opens up a largely untapped dimension of creativity -- at least in the technical LEGOŽ realm.
Perhaps you're thinking, "There's always a work-around with LEGOŽ." I couldn't agree more for static structures and models with only moderately stressed moving parts. However, when you're building machines with a job to do, and you're pushing the envelope WRT the strength and frictional properties of LEGOŽ parts, that's no longer the case. The laws of physics and ABS plastic bow to no one.
My working trebuchet is a case in point. Since its invention in Medieval times, the trebuchet's had but one job -- to hurl whatever needed to be hurled as far as possible. I took that as a mandate to wring every last drop of range out of my treb, and to do so, I'd have to push the mass of the barbell-shaped counterweight (CW) to the limit. Since nothing LEGOŽ even approaches the required CW density, I resorted to a CW made of steel fender washers and lead sinkers at the ends of a 12L Technic axle. To my utter amazement, the axle held its own until CW mass reached 0.3 kg (range ~10 meters). Since the rest of the trebuchet -- all pure LEGOŽ -- seemed to have a lot more range left in it, I then replaced the Technic axle with a 4.8 mm (3/16") steel stud and kept upping the CW mass.
Now, at a maximum range of 20 meters with a 0.48 kg CW, the strength of the all-LEGOŽ beam (throwing arm) has become the limiting factor. Since the rest of the treb is still going strong, it's sorely tempting to reinforce the beam's most common failure points with a few drops of glue. From a safety standpoint, that might even be the right thing to do, but I've stopped for now. The trebuchet may be ugly, but for a 99% LEGOŽ machine, it sure can hurl.