I have only personal experience with 105 and Dura Ace, my husband has a good deal of Ultegra on his bikes.

105 is solid, works and is a good value for the performance, but it can be fiddley when you start getting high mileage on it. My derailleurs started getting hard to adjust after about a year and a half - but I put a lot of mileage on the bike - at least 4,000 miles, not counting all of the trainer time during the winter and the times I forgot to put on the computer or it wasn't working.
My new bike has Dura Ace on it - this was a big jump for me - WOW is all I can say, every thing about it is crisp and smooth smooth smooth. I especially like the brakes - they are much better than the 105's, which were a bit grabby for my tastes.
My husband says Ultegra is right in between, it feels more like 105 - solid though not so crisp, but is reliable like Dura Ace.

some of the differences can be subtle - like the jockey wheels on the 105 rear derailleurs have metal bearings and the ultegras have ceramic ones. My (105) jockey wheels were always getting squeaky because I was riding in the rain alot and no matter how well I dried everything it was hard to keep the grit and water from messing up those bearings.