I don't have Deere's forks, but I made my own. They can lift quite a bit. What I have learned: The first set was built out of 42"x4" forks from a fork lift (very heavy, my setup weighed 300#) was able to lift 600 to 700 with only a back blade for counter weight.
See picture in below post.
42" in front of the loader is a LONG way. Lesson learned! Need smaller forks and less weight.
I found a set of forks in a scrap yard off a hand truck fork lift, like the ones shown in this link:
http://www.wescomfg.com/lift-prod.html
$10 and 5 minutes later, they went home with me. They are 27" long, 3" wide and much, much lighter. I am really happy with this set!!
See picture in below post.
This is really the way to go. I think these forks are the perfect size for the 4100/410. They are big enough for most tasks. The forks themselves had a 1000# capacity on the hand truck, so strength wise, they are well matched to the capacity of the loader. The fork attachment that deere sells, IIRC, weighs near 500#, and the carriage itself is rated for, I think, 3000#. So you buying a lot of weight for capacity you cannot use with a 4100, I won't even mention the price. The only down side, is that I have not made a backstop/carriage as nice as the deere ones.
But to answer your question, they are worth having!!! Not sure if they are worth buying Deeres brand. I use them for firewood (made a pallet with sides, fill it up at the pile and drop it off next to the house), landscaping stuff, moving logs, will try ripping out stumps in the spring (never thought of trying it), moving stuff!! It's just like anything else. Once you have it, you find many things to do with it, and you'll wonder how you lived without it. And if your not handy making stuff yourself, I would guess you could buy the pieces and have someone put them together for you for about $250 or so.
I can provide better pictures and details if anyone wants to undertake this project. Its really a simple build if you have the base skills. I welded it, but it could be done with bolting too. Feel free to email me for details!
Craig