It's good place to stop before going to Belize, because it's cheaper. There isn't a lot to see in Melchor, but can stop at the Yaxha ruin's on the way to Melchor (if you are coming from Poptún or Flores). The view from the pyramids to the lake is beautiful. Melchor is a very little town, it is a good place to see how Guatemaltecs really live.
If you stop there, you have to sleep (or drink something) at Rio Mopan Lodge (on the way to Belize, after the bridge and before the customs, take on the right). The Hotel owner Marco Gross and his wife Inma will give you a lot of very good tips and advises!!
see:
http://www.tikaltravel.com/