I regretted buying my N900 so much that when it finally broke down to the point where I couldn't reliably use it as a phone I went through the effort of bidding for a second hand one in Europe (I'm in the USA) to get my hands on a replacement + extra batteries.

So, negative regret, I suppose.

That said, I can understand some people wanted more out of the N900 than the N900 was offering, and didn't care for the stuff the N900 did great. And some people had higher hopes for a the line of gadgets of which N900 was a part. The Nokia N900 is awesome, but it's currently a dead end. And the N900 is also unlikely to be followed by an equal phone in a different line for a while - some of us got used to the N900 as is, and switching when these N900s are outdated beyond repair and usability will suck, partly because of the actual supperiority of the N900 and partly because getting used to it will make everything else seem worse.

But at any rate, I for one am happy with the N900 (albeit I got it late enough that a lot of the bugs were known and at least some fixed). No other single piece of technology has ever brought me as much happiness since I first got a computer.