I think it is. Because it appeals to the crowd that (involuntarily) missed out on the Nexus 1. Then there is also the "smart phone noob" crowd that will be solely attracted to the screen and HD'2 design while the snappy browsing will not go unnoticed as well. Even the "but it's winmo" crowd is intrigued.
Winmo is a step behind Android, IMO. But Sense gives it a chance to catch up. What you get from Android out of the box, it's scattered across the web for winmo, it's a challenge just to match.
If you are used to android phones or the iphone, once you get over the "new phone feeling", you'll quickly feel "trapped" within HD2's home screen.
That's why I installed SPB's mobile shell. While it's still buggy and not as intuitive as it could be, it's has enormous potential. Should the new version iron these issues out and integrate real time content deeper, it would totally change the experience with the phone. As TMO increases the 3G bandwidth up to 10 MBit/s, this phone will thrive, for me at least.
So I do think it is a flagship phone.