The main thing is the consistency of the quality of the espresso that it makes.  The dual boiler gives much tighter temperature control consistency from day to day and shot to shot compared to my previous heat exchanger machine.  The electronic controls allow you to adjust the temperature based on which beans you are using and I was surprised how much this improved my results. This machine also has enough power to generate excellent steam and to make many shots in a row.  I love the flexibility of using the interior reservoir or directly plumbing it in.  It is also a gorgeous machine.

It's expensive.

There are other lengthy reviews of this product and I don't want to recreate them. I've had no problems with my machine and it has produced much more consistent results than any other machine I have used.  
It is important to note that you need a 20 amp outlet to take advantage of this machine's capacity.  I have used it with Marzocco portafilters based on advice from others at CofeeGeek and I strongly recommend them.
I've had the machine for 15 months now and have used it with espresso from Mr. Espresso in Oakland California and Black Pearl from Chris Coffee, which both produce outstanding results.

I don't know why anyone buys anywhere other than Chris Coffee.  They have raised customer service to an art form.  When I ordered my machine, I decided to invest in a water softener system to avoid scaling in the espresso machine and needed to do some minor plumbing work.  I explained what I was doing to Chris's team and they sent me a customized fitting to make my task easier.  They are always easily accessible by phone and give excellent advice to help you find the best product for your needs based on your budget.  And once you have a machine, they are equally helpful guiding you on any adjustments you want to make.  You can tell how passionate Chris is about coffee and coffee equipment and it makes the whole experience fun.  An expresso machine is a long-term commitment and customer service is very important.