I have one and I have to say that this is one of the best things that happened to puredata..
initially, just by looking at pictures of it and hearing sounds I wasn't attracted to it at all.. Also, knowing how a raspi 1ghz performs (like shit..) I was imagining that the device was too pricey and uncapable of doing something complex with audio..
Then a colleague bought one and When I tested my hardcore patches, I was very surprised to see that they did perform super good and even with very low audio latency.. Also. I have been through the pain of running pd headless in raspi, protecting the sd against writting, enabling usb patch storage, interfacing with display, latency optimization, etc etc.. and I have to say that they did a brilliant job..
This thing performs super good... it's reliable and easy, definitely, it could be cheaper, it could have another design, etc etc.. But if you judge it for what it does, it's the best thing out there, IMHO.....
Also, While I wouldn't pay the sticker price for it. A second hand unit for 3xx Eur / Dollar, (Even the old model) I think is a bargan.. Raspi + additional needed shit gets you close to that number but this would save you lots of headaches...(And probably still performs better)
I did run my patch in a raspi 3 + pisound and the performance and stability were worst (I don't know if is the raspi hardware or raspbian) So I also like to stick with the old Arch / Solid run combo (New units use rp3cm +raspian)
I will release soon a light version of s3rquencer as open source: