Goos simulations and effects quality, good built in materials, good looper precision...

Very good amps and cabinets quality! The HD modeling makes the simulations sound "wammer", if you know what I mean...
The lopper is highly accurated and has a perfect sound quality !
The pitch glide effect, controlled by the expression pedal, is really something you would enjoy to use!
Only the limitation of 3 simultaneously effects makes the owner sad about the POD HD 300... And the LED Display could be bigger... is hard to see from the stand up position...
The built in quality is great! Metal, metal and metal! The expression pedal has an non-skid surface.
The software could be better too... Gearbox software, for example, has a better interface than the POD HD Editor...
For me, this is the best pedalboard for its price! Specially when you're talking about amp emulation quality!

A very good value

There seems to be a lack of reviews and understanding about the the HD 300. The HD 300 is a really nice sounding pedal/preamp/modeler, however you need to take the time to learn how it functions and to some degree experiment with the output settings when connecting to your particualar amp. It doesn't have the bells and whistles of the more expensive POD HD's but it has the same core tones which are very good.

For playing live, once you have a few presets configured the way you want them the the HD 300 is very easy to use. Try setting up one bank of presets (that's 4 presets) with 2 clean and 2 dirty settings. Generally that will be more than enough for an outing and you won't get lost looking for sounds. I also like to set up one bank of effects only presets, in case I use someone elses amps. Again, the trick is to keep it simple. It will help you focus on playing, not tweaking.

as far as the output switches on the back, use your ears to pick the settings, not what is written on them. The line/amp switch works differently with different amps, especially vintage amps. Vintage amps have different optimum input impedances/levels than some modern amps, find what works for you. It also works well with other pedals (in front of it). Yes it has built in effects and they are good, but if you have an effects pedal you really love, just put it in the chain in front of the POD and you are ready to go.

Most importantly, setup your amp and work out your sounds and levels before going out to play. Don't show up at your favorite blues jam with one of these and expect magic if you didn't put the time in up front.

I did not focus on the amp models or effects because that seems to have been covered everywhere in regards to the HD series. The amps are very good. There is something for everyone depending on the type of music you play.

Learn how to edit

I know a couple people with these new HD multi effects pedals and the key to getting it to sound the way you like is to.....make it sound the way you like it...it's pure genius when you really understand that. Get on your computer and play with the settings. It's a lot of work, but once you get your sound, it's going to be worth it.

As for using this live, you need ultra clean settings and some nice speakers. Which can be a down side since your speakers become more expensive, but it's just part of getting the most out of this pedal.

Not that great

I used this device in my studio and for couple of jobs, which was a test to decide if I want to buy it.
I think It is a good device for the money, but it doesn't produce good enough quality to be used for any kind of semi professional, or professional job. Knobs feel cheap, and the whole unit design reminds me a bit on a Roland V88, which is a serious device even today.
Tones sound cheap, and I think Line 6 has to produce better algorithms for their audio engine. The unit sounds thin live, and it is not that big step from previous POD floor units.
I would say you pay more for the design (which was stolen from Roland V88), then for the actual sound quality. Sorry if I offended anyone, just my opinion.

not so much for heavy guitars.... get vox or boss gt10 !

skip this one get vox or boss , . line 6 just does not cut it for me in the guitar distortion / heavy music context.... in the pod hd series nor the xt I had prior.
the effects sound great with bass however with the amp cab mic, modeling turned off. but for that just get the m5 or m9 if all you want are effects from the dl4 mm4 etc
sorry line6 I just didn't dig it. there were more amps and cabs in the floor pod plus, and old pod 2 and flextone heads! vetta 2' and hd147 sound better from what I was told.

ps this review was with a Jackson dinky emg guitar , active.
and schecter extreme bass , and carvin bass.