I got my Helix installed this weekend, took me about 2 hours total, I disconnected my battery and bought the radio removal tools and the terminal tool to make life easy. Only place where I had any real hold ups was getting the CD player back into the dash now that there was some extra wiring behind it.
First reaction, like a lot of people was "well it sounds different!" but different didn't equal good right away. You do have to get used to it, but I'm sure it'll change again for the better once it's programmed. A few things I think have changed, that is affecting the sound the most:
Mid-range and mid-bass have moved. It sounds like most of the lower mids have been moved to the sub, so the sound stage is really high (like 2 feet off the dash, not at dash level like before). Without a lot of that mid fill coming from the doors the tweeters sound like they are doing a lot of the work. I'm hoping that changes a bit with programming, it leaves everything feeling unbalanced because some directional sound is only coming from behind you.
Having never A/B'd a stereo after time correction, I'm just guessing that is one of the biggest changes thats affecting the music now. Echos/reverbs are much more dramatic, and separated, and layered music (think rock w/electronic elements, I was listening to Radiohead's In Rainbows this morning) has very dedicated separation of instruments that the mix doesn't want being on the same stage, if that makes sense. That dramatically changes some songs, not in a bad way (I think) but certain songs that I've become very familiar with how they sound overall have changed a lot.
The gain level at the Helix is just too damn high. I usually never took my stereo over 50% on songs that needed some volume knob. A lot of more compressed music would never need more than 40% or less. Now, I feel like 25% is 40%+, and I just think it's unnecessary. I'd much prefer having the whole volume knob to work with, if I need it.
Also my EQ and faders are zero'd/reset.
I'll post again once I get the unit programmed