++1 So many of these threads get started; kinda fun to read through, but a few irritating themes always seem to emerge

1. "Serious cyclists" don't ride mtb's on road. Sez who? What on earth qualifies one as a 'serious cyclist' -- owning/riding an 'approved' brand or kind of bike? -- or, riding a lot/taking one's cycling 'seriously', and realizing (as per, e.g., Richard Hurst) that each kind of bike (road, or mtb) brings its own set of +/- to the commuting/urban table).

2. "mtb's are sooooo heavy/sluggish/unresponsive" blah blah blah. Oh, really?? Take any decent mid-range hardtail, swap out the susp. fork for rigid, add good slicks, and viola -- a (more or less) 25/26 lb. road-going mtb (or, more accurately perhaps), 'atb' (= French 'velo tout terrain'). Apparently, such a bike is 'heavy, piggish, sluggish, unresponsive', whereas a 25/26/27 lb. steel frame cross, or touring, bike is not And, as said elsewhere here, not that hard to get well under 25 lb working off a decent mtb base.

3. "My roadbike is sooooo much faster than a mtb". Is that so, 'Lance'? Maybe slightly better elapsed time/distance on a long commute over open roads (and/or into a headwind), but otherwise not likely: laws of physics etc. are rather against this one. (We're not talking group rides/paceline here -- those are different conditions). All depends on the rider, and on prevailing conditions on one's commute. I know, beyond doubt, that my blue-pig atb is in fact slightly quicker over my route than my roadie; if my route were different (more 'open'), the converse might well be true -- it all depends. In either case, for MOST OF US, there's not a h_ll of a lot in it.

Here endeth my rant