I bought the vented lowers in Feb. '08 with the quick-detach kit, but have yet to remove them, so I wasted the $80 for the kit. Anyway, they remove easy enough without the extra-cost kit, so unless you'll be removing them every week I wouldn't waste your money.
They keep your legs warmer in winter, help keep rain off your legs and feet (though not in a downpour), reduce airflow from below and thus reduce noise, don't make much difference with heat during summer, and make the bike run marginally cooler when moving (vents open). The downside is that they may increase engine heat while stationary because they trap air (HD's claim), although I haven't noticed this on mine. Storage is not a big virtue of lowers, as the glove box is small and the snaps often hard to negotiate. Lockable doors are available but expensive, and since I'm not using the space I'm considering installing two speakers in place of the glove boxes (Hogtunes kit).