Since I haven't found a post similiar to this, I thought I'd post my
experiences.

I just recently moved and got rid of a lot of the shoplights that I
used to start seeds, and bought a 1000 Watt metal halide fixture.. I
bought it from http://www.hydroponics.net/ mainly because they let you
buy the cheaper reflector with the upgraded ballast.. Other sites (not
all) made you buy the premium reflector with the upgraded ballast.

Anyhow, I have this light hanging from chains in the basement, about 6
feet off the ground. On the cement floor, I have two sheets of 1.5
inch styrofoam to keep the bottoms of the flats warmer. Sheets are
laid side by side, giving an 8' by 8' growing area.. which is perfect
because the "primary growing area" of the light is that much.. the
"secondary growing area" is 12 foot by 12 foot.. but I haven't grown
that many flats yet.

I chose metal hallide over high pressure sodium, because they say HPS
can cause leggy seedlings, and metal halide is supposed to be better
for vegetative growth.

First of all, seed starting is so much more convienent.. no more
messing around with those unreliable shop lights.. And it is pretty
cost effective too, assuming you grow a lot of seeds.
I can put 32 flats underneath the metal halide in the primary growing
area.. more in the secondary. One shop light only has 2 flats under
it.. My metal halide with ballast upgrade was roughly $260 delivered.
Assuming you use $10 shoplights, with 2 cool white bulbs on sale for
$1 each, the cost was very reasonable.. Especially for me, because I
hate those $10 fixtures, and have been buying the $20 fixtures for
better reliablity.. the metal halide bulbs also have a much longer
lifespan.. Not to mention with shoplights, you have to build/buy a
shelving system when you start doing larger quanities of seeds.

The growth under the metal halide has been amazing. Obviously, this is
not a controlled experiment, but I've gotten much better growth.. Like
all my impatients were blooming after roughly 10 weeks (maybe 11
weeks, I didn't keep good records).. All the plants were much larger
than shoplights in previous years.

I strongly recommend this system if you are growing more than maybe 12
flats/year. It's a lot easier to maintain, and gives much better
results. I really see no downside, other than the initial cost to buy
the system, but it should last lot longer than those lousy shoplights.