Price Paid:  $850.00 from Audiogon

Summary:
I purchased this unit before getting the infamous Processor 3 which IMHO is among the very best tube-based DAC you could find on Earth.

I'd say the sound is smooth and very natural. You loose some details to trade for musical sound. But somehow, nothing stands out. You listened to it, you'd like the sound and played more red book CDs on it, but in the end, you won't miss it.

For the $850 I paid years ago, I was STUPID. If you could find one at around $450 today, I think it is a good deal.

Strengths:
Musical, natural, could listen to it for hours.

Weaknesses:
It does almost everything right but really nothing stands out. If you pay $450 for it, it should be an 4 star. But I paid $850, so I have to moan...

Similar Products Used:
Sonic Frontiers Processor 3; Kora Hermes; MSB Gold II and III; MSB III Full Nelson; Classe DAC1; Mark Levinson 36S; etc

Summary:
I changed from the Alpha to the SFD1 only beacause one
became available to me very cheaply. Not to slight either
CAL unit that I've owned, but this unit is much better at
both detail recovery and imaging. I've swapped tubes twice in the month that I've owned it, and have settled on Valvos.
They lack a little crispness at the very top, but more than make up for it in bass. This unit has found a home for a while.

Associated equip:

Sony cdp-x555es
Sonic Frontiers Line One (pre)
Aragon 8002
Paradigm Studio 100

Strengths:
Imaging (L-R and F-R)

Weaknesses:
None so far

Similar Products Used:
CAL Sigma and Alpha

Summary:
I concur with Micheal, adding the Ultrajitterbug and changing the tubes changed the sound from 3.5 to 4.5
in an absolute scale, but for the money paid I would not
change it

Strengths:
Natural sound, details

Similar Products Used:
meridian 203, EAD 1000,...

Summary:
For the money (in the used market), I do not feel you can beat the SF SFD-1 MkII. I have had this in my system for about a year. The mids and bass were a little lacking, but I ordered a set a NOS Semiens 6922's from SF. The change was incredible! If you own one of these, do yourself a favor and upgrade the tubes! The Semiens cost me about $80 (with shipping), but it was well worth it. Also, check into a Genesis Digital Lens. This really improves the does wonders for the overall soundstaging. A Lens can be picked up in the used market for under $1000.
Other equipment:

SF Line 1 Preamp
SF SFT-1 Transport
Genesis Digital Lens
SF Assemblage ST-40 tube amp (a GREAT inventment for $699)
Madisound MDY-4 speakers (check these out at www.madisound.com)
Infinity Modulus Servo Sub

Summary:
Good processor, no doubt. On the plus side: I did not really need a jitter control device to make it listenable, which is something I can't say about Theta Pro Basic II (my regular DAC), or the PDP3. This is not to say that such a device would not help (mine was out for repairs during the audition). Also plus, is great detail, fairly good micro-dynamics, nice treble and nice sense of "bloom".
On the minus side: preface my remarks by admitting that my system is optimized for bass and lower mid-range. The SF unit has very good bass definition, but is lacking in bass and lower-mid weight overall. Compared with Theta DACS anyway, it is a thin sound.
Second minus, macro dynamic contrasts are less than I like. Once having heard what dynamics are possible in a system, one can easily recognize a lessening of those dynamics when it occurs. The diminution of dynamic contrasts with the SFD leads to less listener involvement.
Overall there was a quality that left me less involved. On quieter or more contemplative music the SFD was truly excellent, but when dynamics were in order it fell a bit short. Maybe the SFD2 Mark II makes up for that: I'd like to see some reviews of that processor compared with this.