I like their Excel-like grid control and the hybrid drop-down list / text box. I don't need their support often but it usually has been very helpful. I knocked off a star because they upgraded their products several years ago and didn't offer any upgrade path. Some of our apps have been stuck on that old version because we don't have the time to completely re-code them.

I'm using the tools from Infragistics since a few months and saw many others. I'm still satisfied with this toolset and miss nothing. Although I'm primarily working with Windows Forms and WPF, I also tried ASP.NET and Silverlight for another project recently. ASP.NET is already very voluminous, may be a bit too much. Silverlight on the other hand has only a few controls so far, but they are very powerful compared to others. I'm really looking forward to the next SL release Infragistics announced, there will be a lot more to come.