So I went to this bike shop and asked if they carried slime. I have never used this stuff, but I have always been curious. He says they don't use and don't endorse the product. He didn't tell me why, so now I'm wondering what is so bad about slime

Very messy stuff.

other then that, does it work

Post in the Mountain bike forum.
I don't know of any road riders that use it.

I hate the stuff but it does have some usefulness. If you use a bike a lot and get a lot of flats it may be worth the downsides.
Just don't ever put in in a tire that will sit unused for awhile or you can end up with a very heavy unbalanced spot in your tire.

It works for me. Last Wednesday I was wiping down my chain after a ride and I saw some green stuff on the kick-stand bracket and thought:"Oh No!" I slowly rotated the wheel to try and find the hole in the tire and found nothing. I pumped the tire up to 60# (I ride a comfort bike with big , low pressure tires, max 65# pressure) and the tire is still holding the pressure. I ride on the street and, now and then, I'll pick up something. The slime seems to take care of most of it. I haven't had to patch a tire in over a year now.

Slime definitely works against small punctures caused by nails, thorns and small shards of glass. It isn't at all messy as long as it works since it's fully contained in the tube, or maybe a few drops weep out during the sealing process.

The trade off is that when it doesn't work like on a bigger glass cut, it makes a mess and the tube often can't be patched.

Slime might make sense, if you're getting lots of small punctures, and I think it's a better alternative than tire liners. But if you ride through a glass minefield, and get cuts and slashes you'll regret sliming your tires.

I thought I read somewhere that it can clog up your tube valve, pump fitting, and/or pressure gauge. Anyone had this kind of experience?

I used it in my MTB tires for a few years when I lived in the desert and it worked pretty well against goatheads. I only got one big gash that didn't seal in that time. The mess cleaned up pretty easy with soap and water and I just got a new tube. I probably could have patched it by thoroughly squishing all the slime away from the cut beforehand but figured why bother.

I'm a Mr. Tuffy guy nowadays. A bit more expensive up front but they last forever.

Oh, the clogged valve thing happened once but I was able to clear it by removing valve core and cleaning it. To avoid clogging valves always pump after wheels have been sitting with valves up in the 12 o'clock position for at least half an hour, overnight is best and is easy to do if you park your bike like that everynight then air up if necessary prior to riding.

Oh and on Ira's note about pooling. Yep your wheel balance will be pretty wonky for the first 1/4 mile or so when riding after the Slime is thoroughly pooled at the bottom.