The best in its class for steaming ability (awesome steam!). The largest boiler in its class -- enhances its potential for greater temperature stability. Very well built. Has achieved cult status with a large following on alt.coffee (translates into lots of online help/advice.)
Negative Product Points
Single boiler means you have to wait for boiler to get to steam temp. Built in coffee thermostat has a large range for coffee temp. This means you must "temperature surf" to get a good coffee temp. (Not a huge hassle. See comments toward the end of the review.)
This spiffy looking machine is feature for feature the best machine in its class. It is well made, has a brass boiler, and most importantly, the biggest boiler in its class (approx. 12 ounces). The boiler, IMHO, contributes significantly to its awesome steam power and its potential for temperature stability during coffee making.
I have owned the Silvia for a couple years and recently upgraded to an Isomac Tea (a semi-commercial machine) out of the desire to have something I could: leave on 24/7, and brew shots while simultaneously steaming. The shots are smoother with the Isomac, but with a little tweaking (see comments later) the Silvia produces great shots -- better shots than you will get in all but the finest of espresso establishments. And to be honest, it is more difficult for me to steam with the Tea than the Silivia. There is so much power in the Tea that the milk heats up so quickly. To be honest, I've never achieved the kind of "microfoam" I get from the Silvia on any other machine. The Silivia just seems to be made for steaming.
Perhaps the biggest reason to buy this machine over others in the $300 - 600 range is the depth of information available to you on both www.coffeegeek.com and the alt.coffee archives. There is a steep learning curve when it comes to preparing espresso with a particular machine. Knowing its idiosyncracies up front can save a significant amount of time.
After two years of owning this machine (trouble free) I can highly recommend it. If you do buy this machine, the two most important things you should know that you won't learn from are:
Buy a good grinder. Spend at least $200 on a high quality espresso grinder.
Temperature surf when making coffee. When the machine is ready to make coffee the "wait" light goes out. Trouble is, the water is too hot for espresso. Solution: before putting your coffee-filled portafilter into the machine, run water through the machine until the wait light comes on. Start timing from the moment the wait light comes on. Load your portafilter and be ready. When 35-40 seconds have passed (from when the wait light came on) hit the brew switch. Play around with the range (35-40 seconds) to find out what you prefer in terms of taste.
Some will also suggest (myself included) you get the two cup basket made by La Marzocco and use it in the Silvia portafilter. Maybe it's just me, but I wasn't able to get good shots (consistently) without the La Marzocco basket.