I suppose it depends upon the kind of experience someone is looking for, but I would never visit Guatemala with a tour company. The main reason is that Guatemaltecos are extremely nice people, very helpful and worth getting to know. On a tour, you are unlikely to really strike up the acquaintance of the locals. The major concern that a lot of tourists have there is safety. The safest way to travel in Guatemala is not on a tour bus.
Tour buses are easy to spot, as they are usually white minivans.
Any highway bandit can see them coming a mile away. You can live with a family, or stay in a hotel, whatever suits you. Getting around Guatemala is easy these days, as the roads have been improved, plus the whole country is no larger than the state of Ohio. If you want a guided tour of Tikal or whatever, pay for one with a tour group when you get there. Often I feel that there is not enough debate about traveling south, as most Americans choose Europe as a foreign destination.