I am thinking of purchasing an Infragistics library for a project I'm working on, but I've never used a third-party library in my code before. Is it something you would recommend? Have you ever had any problems with them? What should I watch out for?
Generally good, but I've had exceptions. The best and worst support I've gotten was from commercial software places.
The software will almost always be better than what you would write, and cheaper (if you value development time). It may not meet your needs, but that's something you should check before you buy. If the vendor doesn't have a way for you to evaluate the software before buying, that's a warning sign.
The safest way is to buy software from a company that's in the software business and has been in it for a while. If the company can live off what they make, they must be providing good software and good support. Software from vendors that get their primary income from other things may or may not be good.
Look for reviews, but be aware that reviews are also written by fanboys, irrational people with an axe to grind, people paid by the company (sometimes), and competitors. Unless you get a review from a source you trust, use the reviews to come up with things you want to specifically evaluate.
Consider the policy on source code. Some library vendors include source code, perhaps at extra cost, some don't. The value of source code is not usually so much in modifying it as in recompiling it on different or updated platforms, so it's largely a matter of future-proofing. You may want to inquire about source code escrow.
Finally, don't be too cost-conscious. What would two months of developer time cost you? You can get some good libraries for less than that, and they'll be far better than what anybody could do in two months. If you need it, and it's for sale, it's almost always better to buy than build.
My experience with third party libraries has been excellent.
Granted, I don't know of the specific one you're mentioning as your example. I've used mostly numerical & plotting libraries (IMSL, NetLib, MKL, CalComp, IBM's StatPack) in the past, and in general, they've almost always shown themselves to be better than anything the boys down the hall could patch up.
It's natural really, they live off it (third party libraries company), so it is in their best intentions to make it as good as possible.
Make sure you shop around, and evaluate all the trial versions that are available first. I used an SFTP library and the upload rates between the different libraries varied quite considerably.
I found that price does not neccessarily relate to quality / performance so make use of the trial versions before making a decision would be my advice.
That said, you're usually safe with one of the big names: Rebex, Infragistics, Telerik.....
It depends a lot on the product, some are better than others.
As to Infragistics, they are a mixed bag unto themselves. The quality of the UI components are decidedly mixed. They are fairly inconsistent in API and performance. I think they bought a lot of the components from third party companies and integrated them into their suite, which probably explains the inconsistencies. Also the annual per-developer pricing seemed a bit heavy, especially when MS started providing some of the controls built into ASP.NET.
Note: My information is probably a few years out of date. I've moved on to another team than the one we used it on.