A few things I've noticed....
Using a full range keybaord/PA amp allows you to use the cab sims more accurately. The output mode needs to be studio direct in this case.
If you are using it with a guitar amp and cab, plug into the return of a serial fx loop and disable the cab sims. Plugging into the front end of a guitar amp and cab usually sounds bad, or inconsistant at least.
Use the software to create patches, as 540S said. It is infinitely easier.
Create patches at volume. Creating them at low volumes or with headphones usually results in crap sound when you crank it up.
Disable everything and play through your amp to make sure you are not overdriving it with the input signal level.
Start making patches with the dials at 12 o'clock. Make small changes. One "notch" can be the differance between crap and something that sounds decent. One parameter can affect another, so you may have to go back and forth a little.
Read the manual. I wouldn't take everything in the manual as gospel, but you need to familiarize yourself with all the functions.
The presets pretty much suck. Most other peoples patches, pretty much suck. Like 540S said, maybe 5% are decent.
The upgrades are hardly worth the extra money.
Upgrade the firmware and software on the L6 site.
A
tube power amp warms up the sound and helps get rid of "fizz" at higher volumes.
Be patient. It WILL take time to get the settings right. It's not Plug and Play.
Will it sound exactly like the modeled amp? NO. But you can get some good sounds out of it.
Will it replace tube amps? NO.
Is it a decent tool? YES.
I've had the boss GT-8,
Vox ToneLab, POD, Valvetronix amps and the Toneport. I had the Toneport and GT-8 at the same time. Once I plugged my guitar into the TonePort I took the GT-8 back and got the PODXTL. The GT-8 is great for effects into a nice amp. The 3 cable method works well too. The Vox sounds nice, but forget higher gain settings. The effects on the Vox are so-so. The effects on the POD are just ok.
Mine competes with the Vetta for a fraction of the price.
That is just some of my thoughts.