We did the Appia on foot, on a weekday, but from a further-out point back INTO Rome versus the other way around. If you haven't found the website yet, it will probably be helpful:
http://www.parcoappiaantica.it
There are three major catacombs out there: San Sebastiano (closed Sundays, visiting hours: 10:00 am - 5.00 pm), St. Callixtus (closed Wed: visiting hours: 9.00 - 12.00 and 14.00 - 17.00) and St. Domitilla (closed Tues, visiting hours: 9:00 – 12:00 and 2:00 – 5:00 p.m.)
Here are their websites:
St. Callixtus:
http://www.catacombe.roma.it/en/index.php
San Sebastiano
http://www.catacombe.org/uk_index.html
St Domitilla:
http://www.domitilla.info/idx.htm?var1=docs/en01.htm
Brochure: http://www.domitilla.info/docs/brochures/brochen.pdf
This is reported to be the best of the 3 and one we'll visit next time
Tours are mandatory at all, tickets can be purchased upon arrival, and they sort groups by language so you could have a wait until the next one. We did San Sebastiano and don't recall that we had to cool our heels for an unreasonable amount of time. No, there are no bones to see on the tour route: out of respect, they've all been relocated to an inaccessible level. I suggest bringing a flashlight for peering down side corridors that the tour doesn't enter. I thought this one was interesting for the combination of pre-Christian/Christian tombs, some mausolea and a banqueting room but it's reported to be the smallest and least well-preserved of the 3. Our tour took about 45 minutes or so.
http://www.catacombe.org/text/catacomba%20di%20San%20Sebastiano%20ING.pdf
We had narration from the guide - a little heavy on the Christian piece - at notable spots along the way. The info for St. Callixtus seems to indicate that most, if not all, narration happens before entering the underground corridors. Total length of visit for that one is 30/40 minutes.
Photography is not allowed at any of them, and respectful attire is requested, as at most Italian religious sites.