I take Ticabus because it's not so insanely air conditioned, but some folks prefer to spend a couple extra dollars on King Quality (http://www.king-qualityca.com/
), which is sort of like being on an airplane - beverage service, reclining seats that still work - but with more leg room. There are several smaller international bus companies without websites, but maybe better schedules once you're on the ground. One nice thing about taking Ticabus on the Managua-San Salvador stretch is it's got a cheap, passable hotel in Managua, where they'll wake you up for the 2am bus, and a great $14/night hotel in the Zona Rosa (think Beverly Hills) of San Salvador.
There's no reason to lock yourself into Ticabus for the whole trip, unless you're on a ridiculously tight schedule. Most tourists think major cities are the worst parts of Central America, and that's where international buses stop. For instance, if you want to go to Bocas del Toro, Panama, a groovin' archipelago on the Caribbean Coast about 7hrs by local bus from Panama City, and then cross to some of Costa Rica's best beaches from there, Tica Bus can't do it. They run straight from Panama City to San Jose on the Pacific side, driving right past the best parts of both countries IMO. But you can take local boats/buses between Bocas and Puerto Viejo de Talamanca, CR, a very easy, scenic crossing that caters to tourists. From there, it's a 4hr local bus to San Jose, the capital of CR, where you can hook up with Ticabus again.