Impressed about speed, built-in features like bar code scanning, audible music search, etc. It's honestly rock solid. Still have yet to get a single lockup. Battery life. Integration with Zume Music pass is awesome if you love music. App selection is better. It has everything I truly used on my pre - and then some. The mail app seems to be much more intuitive once you forget about swipe to delete. It handles "Synergy" pretty well although you will have to link profiles a little more than webOS. webOS had a way of knowing who was who and linking where WP7.5 seems to ignore the obvious links. Live tiles are a very nice feature too. And from a design stand point, the UI is minimal, clever, functional, and beautiful. Contact groups are a nice feature too. Group texting, emailing or checking your group's FB posts, etc. is brilliant.
Things I am still scratching my head about are no easy way to close "cards" from the card screen. You have to "back button" your way out to fully close the app. I still haven't figured out how to change the "send as" in emails. I have a number of emails that filter into my one gmail account and I was always able to switch this in webOS but for the life of me, I can't figure it out in WP.
Things I miss from webOS. JUST TYPE!!!! I want to search and not have to go to that app to search for it! I know they are working on integrating contacts into the Bing search but it is not here yet. Same with apps, websites, starting FB posts, etc. This is where webOS just ruled. It was so easy and intuitive and minimized the steps it took to do basic tasks. WP is close but no other mobile OS comes close to webOS yet.
A little funny story from two weeks ago...I went to a friends house the other night and a couple of other friends were there with their new iPhone 4ses and they were demoing some of the new features. I was interested as well. I love tech in general and this is what us geeks do every once in a while; just get together and talk tech. So, my friend asked me to demo some of the features in WP7.5 because they had never used it before. So, I'm going through the gamut of features like the text to speech without even touching the phone, book scanner, text scanner (another one of my favorite features), the Zune music store, etc. I could tell he was liking what he saw and was impressed. So, after a few more minutes of "the demo" he turns to our other friend and says, "Hey Jay, we can't let this Windows Phone show up out iPhones." I knew he was joking but I also knew there were some features he would like to have on his iPhone.
All phones have their good and bad but Mango's pros way outweigh the cons that will eventually be addressed in future updates.