On the Belize side, it's nice...generally organized, new building with clean washrooms, reasonably friendly customs agents. On the Guatemala side it will seem the complete opposite...tumbledown guard shack, surly agents (you'd better brush up on your border crossing spanish), pigs and chickens roaming around, washrooms dirty, confusion reigns with ancient cars, trucks and buses parked and driving every which way.
Even though there is no entry fee to get into Guatemala, you will likely be asked for 10 quetzales. Just pay it and move on. It's only the equivilent of about $1.50US and not worth the hassle of refusing.
Five years ago, when I travelled the road from the Guatemala border to where the pavement started, it was horrible! It took about an hour to go 20 km. Perhaps it is better now.
As far as 'when to cross', the advice 5 years ago was to travel in Guatemala only during daylight hours. Again, that may not be the case now.
Newfoundlanders are the only people in heaven who want to go home.