I've done it off and on my whole life. Truth be told though, I gotta do something with all the meat that stacks up in the freezer. I love hunting too much. My longest stretch as a "strictly" vegetarian was 3 years.
PS, I have more rolls of film than I've had my freezer full... I usually supplement my protein needs with sunflower seeds over my salads, cheese, milk and hardboiled eggs (I know not 100% vegitarian, but I use it sparingly)
I've read a study about the similarities of humans and apes... nutrition wise I mean... and apes are frugavores... not carnivores or omnivores. Humans can live on a diet similiar, but our protein needs are slightly higher. Don't do this cold turkey though. It'd be like a long term heroin addict going cold turkey without methadone. it's not pretty.