We did not use ABD, we used Globus for our most recent trip -- to Italy, but here are some general comments about touring versus going it alone.
1. Traveling when you don't speak the local language is MUCH harder. Not everyone in Europe speaks English. Or is kind to people who do.
2. Going alone you have to do so much research about how to get there, what to see, the history/tidbits about ea location that make it interesting, where to park, etc.
3. Being driven around place to place is a huge advantage. It doesn't matter if you're hot and tired -- you get to sit back and let someone else figure out how to get into/out of Rome. Being shown where to go and not suffernig thru the anxiety of getting lost is a huge advantage. And finally, being shown directly to your hotel and having your luggage transported for you to that hotel in Venice is PRICELESS. I would not want to find my Venetian hotel (even though ours was right off of St. Mark's) or lug my baggage around Venice over uneven cobblestones in 90 degree plus weather dodging peddlers to save any amount of money.
4. You will see so much more on a tour group bec you won't have to get to/from your car that you've parked who knows where -- the bus will come to you. And you also won't waste time standing in line. For ex, aside from a 15 minute wait outside the Florence Duomo, we never once stood in a line to get into an attraction. When the line to get into St. Mark's in Venice was about 2 hours long, we walked right in- - saved 2 precious hours right there. That's worth a lot. When you see the lines snaking around the block everywhere and it's 90 degrees out, having what amounts to a fast pass is hugely advantageous -- you not only see more, you have more energy to go on and see more.
5. Consider what kind of traveler you are. Do you enjoy doing all the homework or would you rather do some and let more experienced people do the rest? Do you get upset and anxious when you are lost or in a hurry to get from point A to point B? Will you get anxious when no one in the train station speaks English? Or it takes you 3 hours to find the train station in Venice and you lost an entire afternoon looking for it? Are you a team player or a lone wolf?
In re Globus, I would give them 5 out of 5 stars. We went on Italian Treasures in June. The group ranged in age from 13 years old to 80. My dd was 14 and loved it. My very-anxious-traveler dh also loved it -- clearly that is his best way to travel since it's the only time he hasn't gotten super anxious when traveling. Lots of walking, so it wasn't designed for older folks, though. Everything was first rate. The only time I worried was when at the last minute the Asissi hotel bailed on Globus and they had to scramble to find nearby accomodations. Our tour guide reassured us, she had a super alternative and since she hadn't misled us yet, we relaxed and 10 minutes later we ended up at the most amazing hotel in Spelo with views that were breathtaking. There was a good mix of touring in a group time and on your own time and they were very helpful about making suggestion for going on your own. The guide was very good at giving you helpful hints about everything inc the all-important "Where can I find a bathroom?' questions. They had good optional excursions -- my favorite was Burano. They took care of us from the baggage claim in Rome to the departure in Rome, beginning to end. No worries, no problems. I feel we got a great value for our money; we got a good taste of Italy -- good enough that if we never go back, we feel like we've done 'enough' to be satisified, but broad enough that if we do go back, we have an idea of what we'd want to do in more depth.