I just returned from a 19-day England and Ireland trip where I used many carry-on and light packing tips that I received from this forum. As a thanks, I wanted to pass on what I learned. I carried a 19" ebags TLS Weekender Convertible Jr. as my main bag and an Osprey Daylite Daypack as my personal item. Here are a few specifics:
Overall
It was well worth it to pack lightly and to be able to easily carry everything. We took several train and bus rides where there was very little extra space for suitcases, and it would have been quite a hassle to have anything large. Having carry-on only also really saved time in making my connecting flight in returning to the U.S. I didn't have to go to the baggage claim prior to clearing passport control.
Ebags TLS Weekender Jr
This was a great bag! I was able to keep it unexpanded the entire time. I used the ebags packing cubes, which really helped me to stay organized. I had done a "test pack" at home and found a packing arrangement that worked well with the bag. It allowed me to keep everything in the same place and pack up quickly. (I brought some extra gallon size ziplock bags to put dirty clothes if they were sharing a packing cube with the clean ones.) The longest I carried the bag on my back was about a mile from hotel to train station and it was quite easy to carry. That said.....if I were to go anywhere where it was really warm (such as Italy), I would have taken my Eagle Creek EC Lync 20" wheeled bag instead. I would have been a puddle of sweat carrying my TLS bag in really hot weather! Something else I hadn't considered about a convertible bag was carrying it in the long airport lines. In the U.S., we were fortunate to be TSA Pre-check, but we waited about an hour in the security lines overseas. That was a long time to keep the bag on my back. I ended up picking it up and moving it forward every few seconds as the line inched forward. Not a big deal, but a wheeled bag would definitely have been easier and I did feel like I got a workout in!
Most Useful Items
The most useful things I brought were definitely the Travelon inflatable hangers, REI microfiber towel, and Forever New powdered detergent. (My husband chuckled at the inflatable hangers until he realized that almost all of our hotels/B&B's had hangers that were attached to the closets!) These items made it really easy to quickly wash items in the sink and dry them overnight. I used the advice I had gotten here to roll the wet items in the microfiber towel and then stamp on it, and that really worked.
Least Useful Item
I brought a travel pillow along that I had tested and liked on car rides. I found that I just don't sleep well on planes, no matter what I bring. I could have used the packing space for something else.
Best Packing Tips
For me, the two best things I did were 1) make sure everything is mix and match, color-wise; and 2) bring at least 1 item that works if you get temperature extremes. The temperatures in both England and Ireland were about 5-15 degrees warmer than forecasted before we left. At one point, there was an unexpected mini-heat wave and it hit the 80's in Ireland (and 93 in London, according to some people we met.) I was so glad that I had packed a pair of very lightweight capris and two short-sleeved quick-dry T-shirts that I mixed and matched with two scarves. I had also thrown in a last-minute pair of Wrightsocks ankle socks. Because it was so warm, I ended up wearing these items way more than my wool socks and long pants that I had planned to use, and it was very easy to wash them out and dry them each night. We met people who were very uncomfortable in the heat because they hadn't packed anything that would work in hot weather.
Thanks again to everyone who gave me so much good packing advice!