Excellent thread. I am a Helix user currently. I have used the HX Stomp/Effects in the past and always went back to a modular pedal system. For me, the Helix has been a great tool. I play upright and electric bass, synths and guitar through my Helix for my Top 40 function band and original projects. For me, the Helix has been a bit of a godsend with respect to routing. I can hook up 4 instruments to it and hand the soundperson a single XLR. Super handy when having to set up a full stage show with lights and a PA. It is also 100% repeatable from gig to gig.
Another benefit I have noticed is that the Helix allows me to control way more with one or two button presses. I can switch to a radically different sound instantly. Using Snapshots allows one to essentially change everything but the order of effects on one button, which I find invaluable. It can also send MIDI to both my external pedals and to my synth to pull up the patches I need without having to make me scroll through the patch bank (and taking me away from actually playing bass).
The sounds are overall pretty amazing. The dirt options are great and the B7k/Timmy models are top notch. I'm not a huge fan of the cabinets, but I also dislike playing with cabinets in "real life" too. I prefer to go direct whenever possible through a great preamp. I've set up all of my patches so that I just take an amp head or preamp model and run it through an EQ with HPF/LPF and found that this gives me the most natural and pleasing tone on bass. FWIW, I do make all of my patches from scratch. The stock ones aren't bad...but they're kind of over the top and definitely need to be tweaked on a per-user basis. Contrary to bholder, I find the mod options to be pretty great. For the few times a night I need a chorus, I'm never disappointed. The phase 90 model is actually one of the best effects in there, IMO. Reverb and delays are excellent, the compressors as well. Octave is fine if you don't need the OC-2 sound. I thoroughly enjoy it, so I ended up throwing a MXR VBO in the loop. The filters and synths are hit or miss...some people love them, others (like me) find them a bit lacking. I use a Broughton Joshwah for that, and just run it in a loop.
The downsides are sometimes I miss the adjustability of individual pedals, the 100% sound of those pedals and the physical sensation of stepping on a box that I've grown accustomed to. There's also a single failure point, where if something goes wrong your whole rig is shot. Granted this hasn't happened to me or many others, but its a genuine fear of mine.
TL;DR:
Helix is great because it gives you unprecedented routing, consistent sound and unrivaled flexibility. The sounds are way beyond usable and if you can't get a good tone out of it, it's not the Helix. Helix is not so great because it takes time and a bit of trial-and-error to get everything dialed in. Also, if you need 100% of *that* tone of something, you'll probably be a little disappointed. Overall, I'm extremely happy with mine, and I'm a stompbox connoisseur.