I just finished setting up a 20Long brackish tank. Right now it's not planted but I have one mollie and 4 bumblebee gobies. They all get blackworms as food once every 2 days. The SG of my tank is at 1.008 and all of my fish are doing just fine! The gobies are extremely entertaining to watch dart around the tank.
To the gentleman that said bumblebee's need a 30 gallon tank, I would like to respectfully disagree. They may be territorial, however the largest of their species gets to be 2-3 cm IIRC, which you should have no problems with in a 20 Long. I agree on the puffers and most other brackish though. F8 would fit (and I'm planning on getting one), GSP's may be a bit too big, but not by much, and monos, scats, and archers etc. will all get to easily the size of full grown angels, which could benefit from a 55 gallon or so. Note that none of this is from experience, but research I've been doing for a few months.
My experience thus far is that the brackish tank is not that much more difficult to operate than a regular FW tank, it just requires a little extra salt preparation, and my choice of fish requires live feeding. I enjoy my tank very much though and I'm looking forward to planting it in the future.