I found Helix Muddy and FM3 not.
I plugged in a FRFR cab to my Helix and Hated it, I plugged a FRFR cab to FM3 and thought I was playing a real Metal amp, Just my experience, But the REV preset in Helix native sounds pretty good...
I demoed the helix and had similar feeling about it.
My overall impression of the Helix is that there is a level of compression that is always on, that makes it sound like its in a box.
Its really easy to play. You turn up the gain and the notes are liquid and fluid, but they achieved this with compression built into the amp sim, and not with normal gain staging. So for a certain level of "gain", the Helix is more clear on top (which to me didn't sound realistic). But if you want the grind and presence of a high gain metal amp, and really turn up the gain/presence, the top end becomes grainy and fizzy, not integrated. None of the amps in the Helix sound right to me. They all have the same "issue".
(Edit: You can actually hear exactly what I'm talking about in the Blix DT sample above. The solo notes are missing harmonic content. They sound smooth but empty. )
The plus side of the Line6 approach is that the amp sims seem to be "over responsive" to volume knob changes. IOW, if you roll back the volume, the front end compression is still hitting the amp sim hard so you still end up getting a fluid (level) sound, which means that the whole sweep of the volume knob is useful. Likewise if you plug in low gain pickups, they will sound full and level without any help. Personally I would rather have the ability to do this myself with a compressor in front, rather than baking it into the amp sim.
The Helix homogenizes guitars and makes them sound more similar. Low output guitars have the fluidity of higher output pickups, and they shave off high end content to make everything sound round.
Overall I think the Helix approach works well for traditional guitars with lower output, but it still always sounds like its in a box to me.
.
