For what it is, I think it is a good product. Solidly built, well thought out and easy to use.
The headphone out is a nice feature which I use often for silent practicing. There is plenty of volume and it's nice to be able to use my amps and effects just as I would live at a gig or in rehearsal.
For recording, I like it too. I've never really been keen on setting up mics at home or isolating my cabs for recording, so again it's nice to just plug into my interface and use my amps and FX just as I normally would to track some guitars.
I've also used it live with great results. Typically I'll use a cab for my own monitoring and run a feed FOH to mix with the rest of the band. This way it keeps the stage volume lower, but I still get to enjoy my amp.
Now, onto the sound...
It obviously is not IR based and doesn't have the sound and flexibility of something like a Torpedo, so if you are hoping that you can get that sound on the cheap, then read no more.
It does capture the essence of your sound, and although I sometimes have to tweak my settings at the amp, or do a little post EQ when recording, I find you can get some nice tones out of it.
I do find that some amps sound better than others through it, and I guess that's because Mesa made it to sound like their speaker cabs I guess? With that being said, I love my Mini Rect through it, but dislike my Memphis through it. With no way of changing speaker types, you are kind of stuck.
So like I said, for the price I think it is a good unit that is nice to have around and gets used often in my case.
For what the Cab Clone does at the price point, it works and sounds pretty damn good. As someone else pointed out, some amps don't sound great through it. Mesa amps sound great through it (big surprise there), so did the Suhr Badger, but I didn't like it with a Marshall or Peavey Classic series.
I will say, that the more expensive, IR based units like the Torpedo sound much, much better.
I've heard decent results by combining the cab clone recording with another IR. It sounded like it had a bit more depth or air to the tone but it was due to the combining of them. Then again, as you said there are plenty of poor sounding demos of the product out there as well.
Glen's "Worst Gear Ever" review (Spectre Sound Studios) was surprisingly biased considering he claims to love his dual rectifier head and 4x12 cab. Clearly Mesa makes decent products. But there seems still to be allot of unjust hate on the cab clone, the cab clone is the equivalent of a sm57 on your cab, nothing more, nothing less, and if you can't get a decent sound out of that, then you have bigger problems. Are there more expensive options with more flexibility sure, but they will cost you three times the price.
For your average musician on a budget the cab clone is a great tool for recording and or playing live, I still do a mic (Senn 609) and blend the two and have had great results. Let alone the silent playing/recording option for late nights. Using in ear monitors the cab clone is a must for me.
It seems like a shock to some players what the direct sound actually sounds like compared to a half stack from over 10 feet away. A little eq'ing goes a long way. Lots of old school guys just like the roar of a cab, as well as eq what sounds best to them, not what would sound best "in the mix". The cab clone isn't excessively high endy or bassy and that's for the better cause your place in the mix is actually more so in the mid range, leaving room for the bass, and other instruments, that's mesa thinking ahead for ya. It's not for everyone, and nor does it compare to high end mics, etc.... but it's not useless as so many claim, especially for the price.