I'm currently building an HK-250GT. My first experience with HK kits was the HK-450 early last year, and in both cases the quality leaves a bit to be desired. You have to deal with QC and minor tolerance issues that simply don't happen with brand-name kits like Align. If you are prepared to deal with those, then they are certainly great value for the money. I would not recommend HK kits to a beginner (at least not one without a "mentor" on hand), because the money saved is exchanged for a more complicated build. Broken parts and weak/stripped screws are common, and some HK parts can be unsafe to fly with (e.g. the feathering shaft on the HK-450).
On the other hand, at that price you can buy 2-3 kits to get a complete set of good parts, you'll have plenty of spares and still spend much less than the price of a single Align kit. Once built, they are great flyers, and are a lot easier on the wallet when you crash them. I bought my HK helis precisely to practice flying, because they are cheap to repair. If you bent the frame or the flybar on your HK, you can just straighten it out manually as best you can, and be back up in the air quicker. I would not risk such "half-repairs" on my more expensive helis. Many of the Align parts will work in the HK version of the heli, you can upgrade to better quality parts where it matters, and as the budget permits.
One notable difference is in the sound they make. I think Align helicopters make an amazing sound as they spool up, and HK helis have a noticeable "grind" in their sound. I don't know what causes this remarkable difference (bearing quality perhaps?), but it's quite obvious. Anyway, I doubt most people care how their heli sounds as long as it flies correctly
Cheers,
Ben.