Not really its intended use, but for simple acoustic guitar + vocal gigs, you can pretty much use an HX Effects unit for your entire front-end (mixer and effects). It works just dandy, and the effects are wow. If you've been thinking about whether or not an HXFX would work for such a gig, here are a few things you should know:
The 1/4" inputs are for instrument or line level. So for a microphone, an external mic preamp would be best - and a must if you need XLR and phantom power.
There is a "Split A/B" function you can use the front of your HXFX processing chain, and a "Mixer" function at the end. With those, you end up with two separate mono paths (A and B). The paths can contain independent switchable effects (up to 9 total, so plenty for acoustic guitar and vocals)
You can mix your vocal and guitar volumes on the fly with a tiny, cheap MIDI controller or iPhone with MIDI app wired to the HXFX, which accepts either 5-pin DIN MIDI or USB MIDI. Be your own sound dude. Of course, you could mix the volumes on the HXFX unit itself, but you'd have to bend over and people might laugh at you.
You can MIDI-control all of the effects parameters, so you can tweak your effects on the fly as needed. Very flexible and very powerful
The HXFX outputs can go straight into a powered PA or speaker(s). You can set up the routing to go out one mono output if you want, and mix the vocal and guitar into the mono out.
The looper is pretty basic, so if you do a lot of looping, you'll want to patch in an external looper (easy to do, or you can patch in any of your other favorite stuff). [Update: after playing around with the new 1-Switch Looper block in HXFX, its likely to be good enough for some folks for simple looping.)
You can get really good acoustic guitar tones. For example, you can use an IR block to make your piezo pickups sound more "woody" and natural. I like 3Sigma's acoustic guitar IRs.
Yeah, there is a three-part harmonizer in HXFX, but don't expect it to do Crosby Stills and Nash. Its not intended for (or good for) vocals unless you want to freak people out.
The selection of effects for guitar and vocals are awesome. The reverbs and delays (the most often used effects for singer/songwriter schlock) are superior. If you've used a TC Helicon pedal, you'll be blown away by how much better these effects are (other than the lack of a vocal harmonizer). If you gotta have vocal harmonies, patch in one of these (or similar TC Helicon box; if you can find an older used unit with MIDI and use MIDI control, get one of those).
You can use separate presets and snapshots for different songs to get a good variety of sounds and tones. The presets are easy to create and edit, especially if you use the Mac or PC editing software.
Line 6 does a great job of updating and bug-fixing compared to most manufacturers, so you'll be supported. And since the pedal is part of the Helix line-up, effects get updated and added over time. Good warranty. Good resale value. Etc.
If you are considering the HXFX only for your acoustic guitar (rather than guitar + vocal), go for it.
UPDATED 2020: I sold my HXFX and got an HX Stomp for acoustic guitar. Why? Line 6 added Command Center support (which was a requirement for me), its lighter and smaller, I can play around with electric guitars with it, and it will have better resale value. Its perfect for my needs