First of all, I must acknowledge that I left the travel business in mid-2002, so some things may well have changed.
That said, here are a few thoughts:
* Trafalgar: well thought of; moderately priced; good itineraries; decent lodgings. Neighbors of mine just took one of their European trips and have been raving.
* Globus: generally well thought of; generally on the inexpensive side; good itineraries; hotels may be away from the town center, not necessarily a Good Thing, especially in Europe; in the business for seemingly forever.
* Petrillo: has a very well-established reputation; specializes in Italy; good itineraries; good lodgings; that said, I've heard some say "They've gotten complacent!"
* Gate One: low-price reputation; the agency I worked in for nearly 20 years never booked them.
* Tauck: One of the best tour companies going, for domestic (i.e., within North America) especially, but also world-wide; brochures carry some high degree of sticker shock, but they end up being worth every penny.
As with all traveling, you've got to weigh the costs against the benefits! On an international tour, you must be prepared to deal with the inevitable language difficulties (a good tour's director knows how to cope); to deal with the probable different standards that you may have with respect to hotels; etc. And, you must include your personal comfort level with the "foreign/strange" or "first-class/third-class.)
Whatever, have fun and take lots of pictures!
SWR