Best bang-for-your-buck iOS guitar interface
I've checked out several iOS guitar interfaces lately, and given the quality and features for the price, the Sonic Port is the clear winner. I tried a few of the IK iRig devices as well as the Apogee Jam. The analog iRig devices (the ones that use the headphone jack of your iDevice) are noticeably more noisy than any of the devices with built-in conversion. The noise level isn't terrible. If you just need a device for practicing silently, then it's perfectly acceptable, but I found that the line-in interfaces just don't cut it for recording.
After deciding I needed something with it's own converter, I spent some time with with the iRig HD, Apogee jam and the Sonic Port. First, let me say that they are all great devices that perform admirably. The "best" one depends on what you need out of an interface. About ninety percent of the time, I'm using the Sonic Port for silent practice or recording, but I also wanted something I could use with an amp. Being that the Sonic Port is the only iOS interface with its own converter that also outputs an instrument-level signal, it won big points with me from the start. It also accepts a line-level stereo signal via an 1/8" inch input and a dedicated headphone output. Add a battery-powered mic preamp and you have a solid location-recording rig.
But, like is said, I'm mainly wanted a high-quality guitar input. The Line 6 works great for this. I love the flexibility of having an instrument-level output, and the input levels are loud and clean. The conversion on this thing is totally acceptable for pretty much everything short of studio recording, and it would probably work great for that too. (Although I still think you just can't beat the sound of a good mic in front of a good amp.)
Listening to the iRig HD, Apogee Jam and the Sonic Port side-by-side, I couldn't noticed any difference in converter quality. I love Apogee products, and wouldn't be surprised if the Jam has a better converter than the iRig or Sonic Port, but you would have to have incredible ears and a perfectly-tuned listening environment to hear a difference. The iRig and the Jam do have a preamp built in to boost the signal of lower-output guitars. This would be a great feature to have on the Sonic Port, but all of my guitars have humbuckers, so it hasn't been an issue for me. This might be something to consider if you're a single coil guy or gal. Also, the Jam can be plugged into a regular USB port and used as an interface on a Mac. It's another one of those features that would be really nice to have, but isn't something I would use very frequently.
The hardware itself seems pretty solid. Not as small as it's IK or Apogee counterparts, but still way too small to ever be considered inconvenient. It has just the right amount of heft to it and seems like it could take some abuse. Of course, you always just take proper care of your gear.
As far as the software goes, sure, Mobile POD isn't great, but don't give the HARDWARE bad marks because you don't like the SOFTWARE. This interface works with any modeling software, and there are tons of great emulation apps out there, many of them free, so don't slam this device just because you didn't like the FREE app that comes with it. You have choices, people!
All in all, the Sonic Port is an incredible, super-flexible device. I gave it 4.5 stars because the Jam and iRig HD have a leg up on it with the preamp, and I would love the option of being able to plug it in directly to a Mac. If Line 6 includes these features in a future model, I'll buy it all over again.