I had a fairly detailed write up on the border crossings from Mex to Costa Rica and the craptacular interweb took a dump and lost it all....
Kind of used to it.
Crossed into Guate at Mesila, easy crossing and painless. Make sure you get your papers taken care of to get your 'deposit' back from Mexico. I used a card and was reimbursed within 3 or so days.
Guate/El Salvador crossing was done at Hacienda Vieja/La Hachadura. Painless and pretty easy. Didn't take that long, but was late in the day when I crossed and drove part of the way to El Gulsnay in the dark.
The El Salvador/Honduras crossing was a pain, almost 3 1/2 hours. Busy to say the least. When you're on the way to the Honduran side take the left fork to the border as the right fork is for commercial vehicles. It's easy to spot the 'fork'. As with any of the border crossings, bypass all the big trucks and get to the front of the line. I'm sure there will be a line. Being patient is the key at this border.
You can go from El Salvador to Nicaragua in day. I wasn't sure if I was going to make it to the Nicaraguan border the same day. I crossed into Nicaragua at Guasaule/Somotillo border. The road past Choluteca sucks as it's pretty cratered with pot holes, as though the road Honduras forgot. Not only are you trying to miss the pot holes, but cars that are trying to miss the pot holes. Fun ride though, but kind of slow going at times.
The Honduras/Nicaragua border was pretty easy and relatively quick. insurance is obligatory in Nicaragua and is good for 30 days. You will be stopped when you get over the bridge.
The border of Nicaragua and Costa Rica is kind of a pain, but not really that bad. You Need 2 or 3 signatures to leave Nica, from the aduana, inspection guy, and police. The Costa Rica side is ok but you need to drive about a block or so past migracion to the aduana which is a little road on the left past an office looking building.
Leaving Costa Rica back into Nicaragua was pretty painless, but still those damn signatures. Make sure you get them or you wont get through.
The border of Nicaragua into Honduras was quick and easy. Came through from Somoto, Nica kind of late and in total took about 45 minutes to an hour including the ride from border post to border post.
Crossed back into Guatemala at El Guarinal to Esquipulas, Gt. Easy border crossing and the only hang up was we showed up around the lunch hour. The bank was closed making for about a 45 minute wait. Aside from that no problem.
As with all of Latin America if you speak some Spanish it makes things easier. Patience and a good attitude goes a long way. I'm currently in Quetzeltenango, Guatemala not sure in a week or so, but send an email and let's have a beer.