I've been working with Helix now for about 2 and a half months. After my extremely positive experience with HD 500 (I have had one since the very first days it was available), I expected good things, but I was truly surprised by how Helix exceeded my expectations. In-depth descriptions of features and capabilities are available elsewhere, so I'll just briefly summarize my experience in terms of what Helix brings NEW to the table, vs. previous Line 6 modelers. 1. Flexibility. This is the big thing right here. 4 totally separate paths that can be linked or not linked, that can work together or separately. For instance, if you wanted to run an acoustic guitar, electric guitar, bass guitar and vocals through this at the same time, with 4 discrete inputs, you can. You can even manage separate outputs for them, some mono and some stereo, if you wish. That's not an ideal situation, because horsepower will be limited, but in theory, it actually can be done. What's more likely is that you will want, perhaps, two separate amp chains, even with some shared FX and some not, or perhaps, as I do, a piezo-equipped electric guitar plugged in, switching between acoustic, electric, or both, each with their own FX, on the fly. I can get about twice as many FX going in Helix as I could with HD 500, and I can get the kind of flexibility that allows me to stay on one patch for an entire set of material if I want. With HD 500, I might have needed 2 or 3 different patches. 2. Ease of Use. Rather than describe this, I will tell you that I'm a manual reader. I read the manual before my Helix arrived. When it did, within about 2 days, I misplaced the manual... for two months... I didn't bother looking for it. It's that easy. Online discussions helping navigate things you wouldn't think you could do with this product abound, too, so get online and learn. It's amazing. 3. Sound quality. I'm going to go ahead and say that I like most of the FX in the HD 500 probably just as much as Helix. Some that are in HD 500 that aren't here (like "Comet Trails") I like messing around with. At this point (early March 2016) I feel like Line 6 needs to add more FX to the box to really "finish it off" (there are fewer FX here than in HD 500), but I can do everything I need here, and I love the FX that are here now. A few of the FX in Helix are vastly improved from HD 500 (I like the verbs WAY better), and some of the new stuff is downright wonderful. The Klon and Timmy gain pedals are simply better than any analog gain pedal I've used, and the Studio Comp is an integral part of every single patch I write. But where we really see the biggest tone improvement in Helix... is... 3a. Amp tones. If you are the kind of guy who plays full-bore, never messes with the volume knob on your guitar, and just needs to be heard loud, you might think the HD 500 sounds just as good, but Helix is more responsive to playing dynamics on the amps I use (mostly the Matchless, /13, Dr. Z, and Fender amp models), you are going to be in heaven. This is as responsive as the best amps I've ever played through. I love taking a clean amp and setting up an EXP to gently toggle to a distorted sound, then add a Timmy or Klon. then back off the volume control on your guitar and it cleans up like (or sometimes better and more predictably than) a real amp. btw. DO NOT USE AMP + CAB MODELS. Use the amp only, then add a dual cab, and have at it. 3b. IRs. I don't use 3rd party speaker cab emulations, but I do use IRs to process my acoustic guitar signal. This has revolutionized my ability to make my acoustic sound great live. IRs already have changed the way I record acoustic guitar, meaning I don't need or want to even bother with miking my acoustic in anything resembling a dense mix. In short, I totally adore this product. One last thing... if you are thinking of getting Helix Rack, but not Control... don't. Get them both or get the floor version. The functionality of the remote with the scribble strips and touch sensitive buttons and on the fly editing with your feet... One of the executives from Line 6 feels so strongly about this that he recommends that if you are buying the Helix Rack without Control, he'd rather you bought a competing product. It makes that much difference. Disclaimer: I am a Line 6 beta tester, but am not compensated monetarily by Line 6 for this review. They nave not asked me to review the product, have not really influenced how much I love it, but you do need to know that they provided a product to me to test.