I recently picked up a Zoom G3, primarily for practicing with headphones. However, the unit comes with tons of features and I'm really happy with the purchase. At $150, it may be the best audio purchase I've ever made in terms of bang for the buck. It has 22 amp sims with cabs, and 94 effects. Up to 6 effects and/or sims can be combined at one time. Those combinations can be saved as patches and there are 99 patch slots available. The unit also has a 40 second looper and a variety of drum rhythms to play along to.
Lots of routing options as well. The unit can connect to your computer via USB for direct recording to your DAW and for very intuitive patch editing. You can also output through L/R 1/4" jacks, and through a balanced XLR output. The XLR out also has the option to bypass the effects so you can record a completely clean track and reamp or add effects later but use the effects for monitoring. As such the unit can also be used as a DI box and even has a ground lift.
The output of the unit can be optimized so it can be used in front of an amp, in an effects loop, with headphones or direct to a mixer, PA or powered monitor. So far I've primarily just used the headphone out and with a Roland CM-30 Cube Monitor. I've been pleased with the sound in both cases. I haven't tried it in front of my amp yet as it still doesn't sound as good as my (mostly) analog pedal board. However, I am strongly considering adding the G3 to my board to replace my reverb, digital delay and chorus pedals. The time shifting effects definitely seem to be better than the overdrive and distortion effects but they are still very usable.
Getting a clean tone with headroom is a bit of a challenge, but that has always been the case with modelers and multi-effects pedals. I was fine with the compromise because my main purpose for the amp was for practicing with headphones. I don't need killer tones for that. That said, I was pleasantly surprised with the sound that comes out of this box. My initial experience several years ago with amp modeling and MFX left a lot to be desired. But this unit sounds good enough to gig with, if need be (not that I gig anymore). I still prefer the sound of my pedal board and tube amp, but the G3 really offers a lot of value.