Slime works great, but you need to make sure you have a way to reinflate your tire after the pressure loss. It has saved me many times from thorns, and glass and such. It is messy if the puncture is too big, it makes for a good show with all the slime bleeding out.
Slime should never be your plan to deal with flats, though, always bring a spare tube or two.
As for the valve getting clogged by slime, it generally happens when you're inflating your tire contrary to the directions on the bottle. It says you should have it in the 5 o'clock position so the slime doesn't run into the valve when inflating. If it does get clogged you can remove the value with the tool the slime bottle gives you and just clean it off.