Very easy to use, and produces consistent, high-quality shots.

Operationally, none.  Aesthetically, it would look a little more attractive with stainless side panels and drip tray, rather than black plastic, but in fairness, the fit/finish is so good that it looks very nice.

We upgraded from a Gaggia Classic that took us four months to master (which included a bit of hacking to dial down the pressure at the OPV valve) before we could pull consistently good shots.  Even so, the Classic can be temperamental, and sink shots were not as rare as we would have liked.

We chose the Mini Vivaldi because of its dual boilers and its temperature stability.   Given the same level of barista quality in our house, our shots became far more consistently good to great.  Even a mediocre pour is drinkable from the Mini Vivaldi.

It is easy to use, and while technically automated to deliver just the right amount of water for either a single or a double shot, those two buttons are programmable.  We like most people online, it seems, programmed the double shot button at the correct water volume given our tamping/grinding style, and use the single shot button to rinse the portafilter.  Steam power is strong (we bought the no-burn steam arm, which is a nice safety touch; it came installed, with the old arm neatly packaged in the box) and steam control is easy.  The hot water dispenser is a nice touch.  We don't typically drink americanos, but it's great for tea or instant soup.  The ability to vary water temperature by a single degree has proven to be valuable.  Black Cat seems happiest on our machine at 92 degrees C, while Barefoot's The Boss seems to prefer 93.  It's fun to experiment with a blend to find its optimum brew temp.

Other reviews I've read say the Mini Vivaldi is sensitive to grind, which seems true.   I've been able to improve shot quality with as little as a quarter-turn on the grinder.

Cleaning is easy.  The machine comes with a bent-handled wire brush.  When you are done for the day, you remove the portafilter and activate the pump, brushing while the hot water flows from the group head.  The oddly-shaped brush handle prevents hot water from running down onto your hand.  After that a few quick backflushes (adding Cafiza to the backflush process every few weeks) and the machine stays remarkably clean.  We're pretty OCD about keeping our espresso machines clean, and this one is far easier and quicker to clean than the Gaggia.  We change the screens every few weeks (it comes with two sets), and the used screens never require more than a minimum of further cleaning to render them spotless for their next use.

My biggest remaining problem is the barista, who is still not as good as I would like.  But I can no longer blame my espresso machine for holding me back, and on those days when I get the grind and the tamp right, the resulting espresso is celestial.

Kudos to Chris Coffee.  This is our second major purchase from them, having bought a Macap grinder a couple of years ago.  I called perhaps half a dozen times over a one-year period before we decided to buy the Mini Vivaldi, and I always found Chris and his staff to be knowledgeable, helpful and happy to take the time to answer my questions completely.  I recommend them highly.

I also appreciated their honesty in recommending accessories.  When I placed my order, they asked me for details about how my wife and I would use it, and our experience level with espresso.  Based on that, they recommended that we not bother with the passive pre-infusion add-on, but that we include the digital timer. They also explained that for normal home use it really was not necessary to buy the 20A version, which would have required a multi-hundred dollar rewiring of the relevant kitchen electrical outlet.  In theory the machine is routing power between the two boilers in 15A mode, but in practice we can't tell.  We bought the digital timer and though it may not be a necessity, it's very helpful when time is short in the morning to know that the Mini Vivaldi will be on and warmed up by the time I walk into the kitchen each day.

Chris packed the unit extremely well--it arrived in a huge box, the original box neatly resealed after unit testing and nested inside a larger box with foam and cardboard braces.  

We haven't needed much support since we bought the machine, but our few questions were handled with the same level of knowledge and patience as were our sales questions.