I use Telerik's stuff daily, and the company I work for has been a customer of theirs for a couple years now.
The "worth" of Telerik's control library is directly related to how quickly you need a project done vs. how much knowledge you already have of WPF. If you don't care about cosmetics or walking outside the standard behavior of the controls built into .Net, then it's probably not worth it. However, if that's not the case, you need to evaluate how much it will take (in terms of time, money and resources taken away from other projects) to build fancy controls in-house. This is the main decision why the company I work for invested in it. It was simply more cost effective to "outsource" the UI components to Telerik. Telerik thrives mostly by providing that gap of knowledge between Win Form programming and WPF that most teams new to WPF do not have.
If you have a solid understanding of WPF in terms templates and styles, most of the Telerik controls do not offer you anything that you couldn't accomplish on your own with a little inheritance and template/style customization.
There are some exceptions though. The grid control, for example, is worth the money. It's a work-horse loaded with features that you would normally take a signficant amount of time and effort to implement on your own.
That aside, there are some gotchas to investing in Telerik. As another poster here mentioned, they generally have good support. I believe the support folks genuinely want to help the people that contact them. However, you will find that they are frequently apologizing for things and offering work-arounds. I've seen a few situations where behavior bugs are re-introduced every other release. Sometimes I wonder if they are so eager to appease their customers, they find themselves in a loop of breaking code for one customer to fix something for another.
They release quarterly, so you are held hostage to bugs until those releases. And often times, you will feel like you are "free labor" to their beta testing because your investment in their controls demands it.
Upgrading their components is a game of russian roulette. Always back-up and have a roll-back strategy when you upgrade their components. Be prepared to spend a few days fixing things that break, or working through your customizations to eliminate any weird behaviors that are introduced. If you minimize the amount of customizations you stack on top of their controls, generally the smoother the upgrading will be.