Along with the standard Android widgets, HTC has included a handful of its own (which are as nicely styled as the rest of the interface). Our favorite among the HTC widgets would have to be the Friend Stream, which allows you to view all of your social networks in one unified stream and gives you the ability to post something simultaneously across your social networks. It's much faster and easier than having to go to each individual app and update your status multiple times.
Another useful feature is the ability to pinch the screen and get a bird's-eye view of all your home screens. Still another awesome widget is the weather app, which accents your home screen with fun animations depending on what the weather is like outside (raindrops and windshield wipers when it's raining, for example).
HTC Sense's Address Book is also an improvement over Android's address book. When you tap on a contact, you can see all of their recent activities and Facebook and Twitter updates, as well as the texts, calls, and e-mail messages between you and that person.
One thing we don't like about Sense is that it can take a while to update your social streams--especially if you have a lot of people in your contact list.
Overall, however, people who are sick of the plain-looking Android interface might do well to consider an HTC Sense phone.
Although these overlays can be very helpful to certain types of users (newbies, social-media types), it's important to remember that handset makers have their own reasons for putting their own software over a phone's OS. The interface of a mobile phone is valuable real estate, and handset makers want their own brand there when possible. Because OS overlay software can be such a large part of the user interface, you should consider it when choosing a phone. If it might help you do the things you normally do on your phone, great--but if just creates clutter, you might be better off looking elsewhere.