We have used ComponentArt extensively in one of our projects for years now. They are useful up to a point, as every now and then, there will be a requirement, that is just not possible to implement using the existing API.
Having used Infragistics as well in another project and now working on one that uses Telerik, I can tell you one thing: Spend some extra money and buy a control suite with the full source code. Sooner or later, you will come across a limitation, and having the source code with you will be a great help.
Of course, changing the source code has the downside of it becoming obsolete when a new version arrives, but it is still better then requesting a feature from the control suite vendor and then hoping that it will be part of the next release.
