Simply put, the tracking, stability, and general handling of the boat is good. Not great, not bad, and very comparable with most kayaks of similar size and design, even the more modern ones, like my wife's Perception Calypso. You're splitting hairs to find much in the way of performance differences in that regard, and the average to experienced kayaker will find the paddling experience good IMO.
I give the Willow extra points on its hatch cover. It's one of the more robust designs you'll see in a small kayak, with a rubber friction cover in addition to a solid plastic cover with two buckles. Its sea-kayakish in design and impressive. Combined with a rudder, you've got yourself a poor mans sea kayak
The kayak definitely looses points in its seat construction. Very little materials and build quality were put into the seat. You WILL need to upgrade the seat by replacing the seat or adding an add on seat system with cushions etc. I can endure almost anything but this seat becomes downright painful after about a half hour.
Finally, and almost unfairly, there is little to be gained in weight savings from this light Kevlar kayak to its modern counterparts, even in plastic boats. My Calypso weighs 38lbs. The Willow weighs 36-37lbs depending on which archived information site you reference. Basically, there's no difference. What you do sacrifice with the Willow is durability...
By now if you're in the market for a Willow pay close attention to the hull and spider cracks. Most of these are superficial and can be painted with marine paint and do not compromise the hull integrity. However, unless the kayak has been well maintained, you might not want the headache of purchasing an old kayak that needs touch up work.
In the end I'd say that if you can find a Willow really cheap go ahead and pick it up. I got mine for $250 on craigslist. Originally it was a $1500 boat. Anything approaching $500 or more I wouldn't say is a great purchase for this aging and average kayak.
In conclusion, the Willow is worth the price if you want lightness. Frankly, no one will find just one kayak to suit all their needs. After a period of time most kayakers end up buying several different boat for various wants and needs.