We have questions about free .NET libraries and applications, but I'm curious about what ASP.NET control packages you've used that provide the best bang for your buck. Yes, paid stuff.
I'll get the ball rolling by saying I love the Telerik controls, but their price tag isn't great. The one reason I would say the value is the greatest here because of the licensing, which makes deployment so easy.
Telerik is by far the most robust for the cost; but you're right that the cost is a bit high.
We use the "Web.UI" suite from ComponentArt and we're very happy with it. You get the source code, redistribution, etc. Makes our web app stand out for sure.
I have both Telerik and DevExpress and I find myself, more and more, moving away from the Telerik stuff and using more of the DevExpress controls. I particularly like the DevExpress reporting package (the Telerik one was unusable) and the Grid / PivotGrid. However, Telerik does have better AJAX management helpers (although I no longer use them as I've learned enough about "raw" AJAX) and the Telerik Editor control is the best there is.
So...one vote here for DevExpress.
[Full Disclosure: I work at Telerik.]
I won't make any statements about which controls are the best- since clearly my opinions would be biased- but I will say that in the "3rd party" component market (in other words, not Microsoft and not open source) the pricing amongst the big players is all about the same. And in general, if you need the rich functionality offered by any of the 3rd party suites, the price is significantly lower than what you'd pay in your own dev time to rebuild the controls from scratch (or even to customize an open source control).
There are some significant differences in the way the licensing is handled, though, and as Josh originally highlighted, Telerik has the most open, "developer friendly" licensing approach. Purchased controls have no license keys to deal with.
Hope that helps.
The cost of Telerik's / DevExpress controls are CHEAP compared the cost having one or two people develop "something better". I have had developers work two weeks on just a grid with no Ajax, no client side anything and it looked like crap. Unless you have great skill with tools like jQuery and the team all possesses those skills, it is cheaper to go with a purchased library. Stuck on an issue, open a ticket. It's that simple.