I used Developer Express .net controls but got rid of them and have just started using Telerik. I can't speak for Telerik as of now but I was not fond of Developer Express' .Net components. That being said, their Delphi components are awesome.
When I purchased DevExpress' .Net components, I had researched many components and their reviews and Telerik and DevExpress were high on my list. I chose DevExpress for two reasons. One, I had previously used their Delphi grid and it was great, and two, they had many more features within their component suite than Telerik.
Unfortunately, the fact that they have many more features only means that they have more code to support and more bugs to fix. A previous poster complained about Telerik's release dates. I can tell you that the same problem exists with DevExpress but their releases fall more frequently than per quarter. DevExpress' marketing scheme is to continually add new functionality and gadgets. However, their frequent releases only adds more bugs to their code. It was exciting to have bug fixes released on a frequent basis but in reality, the new functionality that they on top of the bug fixes broke other things.
I was constantly having to rewrite code blocks because either function parameters changed or were deprecated. Their support was just o.k. but not anything phenominal. I got frustrated with one support tech because I wanted to know how to color a grid cell for field validation. He actually argued with me and tried to convince me to use a label to input my error messages into--this didn't even answer my question and wasn't what I wanted. Finally, I got him to answer my question and found a way to do it but it was like pulling teeth.
My biggest beef with them is that I had four projects that used their grids and one of those projects used their reporting feature. The amount of bugs that they had in their (asp.net) grids forced me to scrap them and use asp.net's gridview instead. I was hoping to salvage the reporting feature but that had a bug in it as well. I fixed the bug in their report code, reported the problem and gave them the solution, and they basically denied that it was a problem. However, my fix was in their next release without even a thank you.
I am now onto Telerik using winforms for a quick project. It's been easy thus far. I've used their panels, tab control, and soon their grid. I'd use WPF but there's a learning curve with WPF and it's something I'll have to get to later.