Took a while, as I have other infrastructure changes/upgrades in flight...
Seems to work ok, the big deal with the SlingBox M1 (and subsequent M2) is the WiFi connectivity - and that generally works. Note that WiFi is a single stream radio - ethernet is also available, 100Base-T, and is recommended.
First run wasn't too good, but second attempt with Component connections between the DirecTV C41W box and the TV worked much better... still a bit of latency across clients, and the C41W is basically a remote screen for the HR44 Genie box, so the lag adds up - just be patient.
On Mac/Windows - it's a browser plugin (boo!), and as such, not a terribly good thing.
iOS - iPad/iPhone - uses a new App, so if legacy, remove the old one first (I had SlingBox Pro, see other post), but after some sorting, it works...
Would I recommend it - maybe, maybe not - I'd give it 3 stars out of 5 due to setup and config issues...
Where it can be a benefit though - we see on the forums many times where folks are just out of reach of Broadband, so running internet connectivity is one thing - either trenching a ethernet cable, or running a wireless bridge - that gives broadband - putting a Slingbox on the remote end, and using Desktop/Roku/Android/iOS apps can bring TV into that remote location.