I did a group tour of Ireland several years ago and had a blast. But it is a very different type of vacation than the kind I usually take. You just have to keep that in mind.
The pros: We had a driver/tour guide who was phenomenal. I think that was just luck, but the guide will have a HUGE impact on your vacation. He was a great source of knowledge but was also just really funny.
We got to see a lot of the countryside that you only see by driving yourself, except you don't have to worry about the driving. It was very relaxing just riding the bus around from place to place. Our guide was really fantastic about stopping to take pictures of random stuff we thought was photo-worthy (spray painted sheep, for example).
You get VIP treatment at a lot of places. We didn't have to wait in lines. We got private tours.
We didn't have to worry about accomodations at all. Most dinners were also arranged for us.
I was worried that we would be very separated from the actual Irish people - traveling in a pack. However, that was never a problem. We met lots of people at the hotels, restaurants, places we visited.
The cons: You really don't have much say so on what happens during the tour. You are just along for the ride. For example, we missed one site because road construction was taking us too far off of our schedule. On a personal trip, you could adjust your plans. Not so much on a group trip. We were also not given enough time at a few sites. It's all about the schedule.
You don't get a say-so in most of your activities. We did have a lot of free time in the cities, but most of the places we visited were scheduled stops. We did a LOT of monastic ruins on our trip. After a while it got really old (not a pun). But we were in the middle of nowhere, so it's not like you could go do something else.
We stopped at a lot of places that were interesting, but were definitely geared to sell stuff. We did a lot of factory tours (here's where they make bodhrans, here's where they make Galway Crystal, etc.). It was wonderful seeing the craftsmen at work, but we were given long amounts of time to peruse the gift shops. I didn't buy much stuff there and instead saved my money and bought the same items much cheaper at a large department store in Dublin.
You tend to stay in hotels that are geared for big bus travel. They aren't in the center of town. A lot of times we had to take a taxi at night to get anywhere. Most of our dinners were these extravagant three-course meals that we could have easily done without to leave more room for sightseeing. Of course, we were doing a lot of walking around, so the hearty meals were appreciated, but I enjoyed eating in the pubs and local restaurants more than the hotel dinners. The hotels we stayed at were all wonderful, though - very good service.
Overall, if I had to do it again, I would do the exact same thing. I think it is a wonderful option if you want to cover a lot of ground in a couple of weeks. You would have to have a rental car to do most of the things we did and that has its own set of problems and headaches. We are considering doing another group travel if we get to go to Egypt.
I wouldn't think it was worth it if you just wanted to see a small region or city. You can easily hire a tour guide for a day to see some of the major sites. I would suggest adding days onto the end of your trip. You usually end in a large city and it's a fun way to end the vacation doing what you want to do and perhaps getting to do the things you are more interested in.