Dual boiler, rotary pump, powerful heating elements
Very stable electronic temperature control, can be set in 1degC steps
Large steam boiler (2.5liters), works well for micro-foamed milk
Fast start-up time (less than 20 minutes)
Volumetric shot dosing (can be stopped early for 'semiautomatic' use)
Steam boiler can be switched off (for straight Espresso enthusiasts)
Large drip tray
Very solid built

Plumbed in, but not drained out.
Bent steam arm somewhat short for 20oz pitchers or more

OK, first of all, there are a number of great reviews out there for the S1, so I have no intention of reiterating too many facts here. If you haven't done so yet, go check out Dan Kehn's review on http://www.home-barista.com , or Chas Rimpo's website at http://s1.rimpo.org , where he dedicates a whole website with forum to the S1, and see various highly rated reviews for it here on coffeegeek.com.

That said, I wasn't planning on writing a review at all, but I just pulled another glorious Espresso shot and started typing…

The S1 is definitely one at the very high end of prosumer Espresso machines. It is well built on a solid steel frame with quality parts and except for side panels and drip tray you won't find much plastic on it.

One of its key features is a very stable temperature control which enables the barista to pull on consistent shot after the other. At some point I was wondering that even though it has electronic temperature control, it doesn't have a PID. Then I started measuring temperature profiles and comparing them to other machines out there. The result was, that after the L-curve at begin of extraction (4 to 5 seconds into the shot), all my measurements showed less than +/- 0.5 degC (1 degF) maximum variability until the end of the shot (25 to 35 seconds). And I could reproduce it consistently (with a Fluke 54-II and Schomer's measurement technique). Its double boiler design ensures temperature consistency even it you use the hot water spout or steam while pulling a shot.

The temperature can be comfortably set in single degree Celsius on the front keypad from 82 to 123 degC (85 to 120 coarse steps, +/- 3 deg fine tuning one each coarse step). Since the machine has such a great temperature consistency, I sometimes wished one could set half degrees (or one Fahrenheit steps), to further fine-tune my shots…

The Fluid-O-Tech rotary pump does a great job creating consistent pressure. One thing you might want to buy would be a PF pressure gauge to be able to check for the right pressure (I have mine at 8.5 bar), since the built-in gauge is for the steam boiler only.

The PF itself is very heavy and cleared all doubts I had before about buying a 53mm basket machine. Just get yourself a good tamper (Thor-Tamper, Reg Barber, … I'm becoming a tamper collector…).

One critique remains, since my machine is plumbed in  I'd like it to be drained out too (wouldn't that make sense?). S1 is working on a drain option, but the one they came up with as to date, is (according to Chris Nachtrieb, the General importer for La Spaziale in the US) not really practical yet, so I'm still waiting. On the upside, the drip tray is quite large.

The steam boiler has a generous 2.5 liter. No problem getting that micro-foam for latte art (I'm still working on my skills though); only thing I would have changed is the length of the steam arm: Dan Kehn and other sources also mention that using a larger pitcher than 20oz (that works well for steaming 10oz of milk) is unpractical as a result of a bend in the steam wand. I typically use that 20oz steam pitcher that works well for two cappuccinos.
The hot-water spout is good for rinsing the PF or making a cup of tea (and yes, the water is hot enough for tea, I measured 95.5 degC, 204 degF in a plastic cup with my Fluke54 thermometer).

For those of you that like to turn on their machines with a timer in the morning, the S1 can't do that (without serious modifications) as it only goes to standby as power comes on. On the upside it only takes about 15 to 20 minutes for it to reach operating temperature.

Oh yes, if you are thinking about getting the 15Amp model think twice and get the 20Amp model, if you are planning to use steamed milk with your coffee (otherwise you will have to wait longer and who wants to wait! Better have a 20Amp circuit pulled in). On the bright side, every 15Amp model can easily be switched to a 20Amp model (internally) and each 20Amp model can be switched to Econo-mode from the front panel, which limits the current to less than 15Amps (by not heating both boilers simultaneously).

One thing that's a real necessity having the plumbed-in model is a water filtration system. In areas with hard water I'd recommend a water softener and purification system, while in areas with soft water I would still recommend the purifier (you never know what comes through your municipal water pipe, and any little debris might damage the pump rotor or gets the electro-valves stuck). If you like to ponder about water, see: http://www.big-rick.com/coffee/waterfaq.html

If you go to Chas Rimpo's site you'll also find a triple PF basket, that I found absolutely great with some espresso blends. For custom fit tampers and wooden PF handles visit Les at http://www.thortamper.com . Also the naked PF (like found on Chriscoffee.com) for the S1 is a great tool to use.

Bought at Chris-Coffee ( http://www.chriscoffee.com ). They are absolutely great to do business with. No matter if I made a purchase or called in for a service question, they showed superior knowledge and customer service, and answered all my requests quickly.