I like the idea of low voltage recesssed halogen lights but my local hardware superstore stocks mostly line voltage fixtures. Are the low voltage fixtures unpopular for some reason?

No, in fact I prefer the low voltage tracks. The difference is aesthetics. With line voltage systems you must use a 110V bulb which I think comes in only 1 size or you must use transformers at each lamp fixture on the track, this makes them bulky. Also the tack must be a "U" shaped track to protect the line voltage conductors inside. With low voltage tracks you get much more design flexibility, the tracks can be wires, tape or track. The lamps can be anything the imagination can come up with. In Europe all the stores have gone to low voltage halogen display lights. Mostly with am MR16 bulb hanging down on a thin stem.

You actually get stronger light from the low voltage lamps! In addition now that the transformers are located right at the fixture it is so easy to install them. I'm in the process of installing the Lightolier 3 1/4" recessed cans and they are great -- small, adjustable, easy to install. Can't wait until I'm done!