I bought the DevExpress and Telerik libraries, and although both are good, I ended up uninstalling Telerik and sticking with DevExpress for some pretty specific reasons.
Firstly, there are many overlaps between them, and they really share a leading position in the ASP.NET/C# market.
DevExpress has a significant advantage with respect to its reporting tool. No tool I rated was perfect (I looked at four overall), but DevExpress was the easiest to use, easiest to deploy, and at least as powerful as any other (with the possible exception of , MS tool), but which has its own deployment problems). This was a big advantage as reporting was also central to my application.
DevExpress graphics library is very nice and integrates well with the reporting tool - another victory for them. Dundas and Telerik have good libraries, but, after many years of work on diagrams, simplicity of configuration / definition is important for me, and DevExpress has an advantage there.
Telerik has the best online HTML editor (we have a CMS in our product), but the DevExpress product is not so far behind. If I hadn’t been involved in reporting / scheduling, I could go with Telerik only on this basis, as our CMS is very important, but this is not enough if you are creating a report / chart.
Grid products are excellent from both, but DevExpress also offers PivotGrid, which is an impressive data analysis tool and a good addition to any site where it is important to analyze data.
Both have very good support in my experience.
The common controls from both are very capable, but to be honest, there is little reason to choose one for the other controls that concern me. My focus was on controlling date management and numerical inputs.
So, I absolutely understand that people are happy with one of the two, but I ended up (after using both for two years) coming only with DevExpress. It didn't hurt that the DevExpress team also wrote a book that really helped determine the range of use to which their library could be added.
Hope this helps!