2:50 p.m. on November 18, 2007 (EST)
Since you are going on a guided trip, your guide service should provide you with all the information needed. You might also want to look at RJ Secor's book on the Mexican Volcanoes (get the latest edition, since he adds a lot of updates to each edition). And, since you are going with a guided group, you won't have a lot of spare time (unless you go early or stay extra time - which means you will miss most of the Mexican culture, like the Pyramids, Guadalupe, and the really fascinating museums).
Presumably, you will be doing one or more smaller peaks before Orizaba, such as Iztaccihuatl, or maybe even La Malinche (smaller, but reasonable for acclimatization). And presumably you will be staying in or close to the Piedra Grande hut the night before. Hopefully you will not be sleeping in the hut itself (it's noisy, smokey, and has lots of rats and mice, despite the efforts of the guide services to clean it up). And hopefully, you will have an intermediate camp at 16,000 ft. The glacier has retreated a lot since my first climb of the peak, so you will probably have several hundred feet of climb before you need crampons. And it does get windy, so you may have a lot of the volcanic ash that passes for very fine dust, blowing into your tent. If you are lucky, it will snow, keeping the dust down.
You aren't likely to spend much time in Tlachichuca, but there are several internet cafes there (really cheap compared to anything in any other country I have been to). You will probably spend more time in Puebla. If so, be sure to visit the big cathedral - huge and gorgeous. Instead of having breakfast in the hotel, go to Vip's (this is a chain, like Sanborn's). Since you will probably stay in Mexico City, have dinner at least once at Sanborn's La Casa de los Azulejos ("house of the blue tiles"), though the touristy souvenir section is a bit much.
If you do stay at Joaquin Conchola's place in Tlachichuca, say "hola" to him for me - he is an old friend from many years back, though it's been 4 years since I have visited. Joaquin has been running one of the main 4WD runs up to Piedra Grande for many years. And be sure to admire the huge trophy he was presented by the State of Puebla for services rendered.
Experiences - mainly it is just a long slog from Piedra Grande, first from the hut at 13,000 ft up the trail, alongside the aqueduct (I would be curious if it has ever been repaired), then on the mostly rocky trail. There are a couple places where there is often ice in the couloir where the trail goes - use care there, and maybe use your crampons for that section (it isn't long, and the guides may not bother, but I have watched people take a long slide through the chute - depends on the conditions and how experienced at walking on ice in your boots you are). The area around the campsite at 16k is pretty filthy - there is no latrine at 16k, so you may find it hard to find a place to go between piles of "droppings". (the latrine at Piedra Grande is in the open, and people frequently just relieve themselves in the gully below the latrine). Hopefully, since you are going with a guide service, they will provide WAG bags, though you will probably end up tossing them into the "relief" area.
You will (or should) get up early (pre-dawn) to head up the hill. Again, it is a long slog, now on crampons, to the crater rim. Drink lots of water and eat frequent snacks. Once at the rim, you traverse around and up to the cross. Take lots of film or a big memory card (and keep your camera in a plastic bag inside your parka to keep it warm, or attach a handwarmer to it - batteries get weak in the cold and electronic cameras frequently fail in those conditions). Then it's down (time to use lots of care - I have caught a couple packs that people have dropped and watched people take slides due to being fatigued and suffering from AMS). Then you pack and head down to Piedra Grande (watch the ice on the trail, again), get picked up by your driver, then back to Tlachichuca, and probably on to Puebla or even back to Mexico City.
Not much to it. Typical glacier slog. Great views, though.