Do you have a good experience with a control library? Something that is kind of robust, well documented, consistent (across different controls) and quite well integrated into the Visual Studio.
Well, I can only speak about the Infragistics controls - they have a lot of bang for your buck and are well documented, very consistent and are well integrated with the standard ASP.NET programming model, etc.
Begin rant:
I personally think they are bloated and past their prime in today's world of lighter-weight JavaScript libraries and toolkits. Most developers are becoming more and more proficient in such toolkits, so the abstractions provided by Infragistics and other such and similar control vendors are not needed as much.
But that is purely my opinion.
ComponentArt has some pretty cool controls. You might want to check out Telerik as well. Both companies offer pretty easy to use controls that look nice.
I've looked at a lot of control libraries ... too many to count. I like the DevExpress controls as they provided a complete suite for Windows and the Web. They also include charting, gauges and reports. We write apps for Windows and the Web so it makes it easy to transition between the two.
Though, when it comes to a Web environment we try to minimize the custom controls we use, just because of the added bloat.
I'll second the vote for Telerik. Their controls for the most part "just work" and their support has been excellent. I primarily use their forums and I still receive a response within a day (unlike some other vendors who barely seem to notice that they've even got a forum).
It also feels like they've actually spent time trying out a lot of the ways customer's will use their controls. The documentation and support reflects it. They aren't perfect, though. One issue that they had in the past, and that they've addressed in the latest releases (what they were calling their "Prometheus" controls, now just "Rad Controls for ASP.NET AJAX") is the performance of the controls. In previous releases they were definitely a bit sluggish (I'm thinking specifically of their RadGrid and RadEditor). Now they're noticeably faster (esp. the RadEditor - it loads MUCH faster).
Overall I wouldn't think twice of recommending them.
I don't have a recommendation, but I do have some feedback on the Telerik recommendations. I can't stand their tools myself. The performance of their more complicated controls (e.g., Tree, Grid) is very sluggish and feels very un-web 2.0.