sense is more of an "official android ROM" form HTC in my opinion.
It's hard to say at this point. I think two points that other posters brought up earlier in this thread are relevant to a degree: 1) This is the first new Sense UI phone that has the specs to easily run Android 2.X versions; and 2) Android 2.1 has been ported with Sense UI, so other 2.X incremental updates can probably be optimized more quickly.
But the two above points are still somewhat speculative. The safest assumption to make with this phone is that updates will be delivered in a delayed manner, or possibly not at all. After you make that assumption, you have to ask yourself if the benefits to having Sense UI make up for the detriment of not getting timely Android UI updates.
At this point, most of my issues with Android 2.1 are merely cosmetic and integration issues, so I'm very happy to get Sense UI. I used to be a strong proponent of getting the strict Google Experience and previously had no desire for Sense, but I don't see any major updates coming down the pipeline until 2.5-3.0, so I'm not worried. I'll likely have a new phone by that time anyway.