I have been vegetarian for three years, and haven't taken any supplements consistently throughout that period of time. The last time I had a blood panel done, I didn't have any mineral deficiencies, at least. I am by no means against taking supplements, but my impression is that they aren't fully necessary for vegetarians who have a well-balanced diet.

I did take B12 for a few months when I was experimenting with reducing my intake of eggs and milk, though I eventually decided that I really liked eggs and milk, and consequently stopped taking B12. I've recently started taking CoQ10 because RomeoStevens advocated doing so here.

In the past couple of years, I have considered becoming flexitarian (i.e. 98% vegetarian) or pescatarian, mostly for convenience and health reasons, respectively, though I've elected to stay vegetarian for now. This is partly because I'm used to being vegetarian, partly because I've accidentally built vegetarianism into my self-identity, and partly because of the normal reasons people give for being vegetarian (health, environmental, and compassion-towards-animals type reasons).

Added 6/29/2015: Apparently, I haven't been getting enough fiber for at least the last couple of months, but that is due to me being lazy about my diet, rather than any shortcoming of vegetarianism.