I have never been able to design something decent enough to promote in the various design galleries, but I have witnessed first hand what happens to a site that is widely promoted in such a way.
If you have what you consider a beautiful custom design for your site, the rewards of taking time to show it off can be huge if done well.
I recently read a post on Design Reviver about promoting your site on design galleries, and they give some great advice. I used to work on posting sites on CSS Vault, one of the first CSS galleries and also one of the biggest, and highest PageRank ones, and the tips in the article definitely held true when I was working on CSS Vault.
Submitting with a Purpose
You could randomly submit your site to one gallery and more than likely end up on many more. However, if you take a more structured approach to your submitting, you end up getting featured on more sites. There are some galleries that want to be the first to feature you, and probably won’t feature you if you have already been seen on too many other sites. The key is to submit and get listed on these first, then follow with the less picky sites.
If you aren’t sure if your design is gallery worthy or not. It doesn’t hurt to submit it and see what happens. The worst that can happen is you aren’t featured, and have lost a little bit of time, which is not a big deal when you consider the possible rewards.