I have the Wiim Ultra and not the Nano. I use an external DAC but two things that made me go for the Wiim are the software which is fantastic. Easy to use from your phone or computer, navigation is great, has room correction/EQ. I went with the Ultra because it allows me to connect a USB drive/HD and run my songs off of it like a server. And the software is always updated and they listen to customers.
The built-in is DAC is good but a nice external DAC makes it sound a lot better. And its a damn good price especially if you catch it on sale.