I recently upgraded a pair of Solo's to Pro-HD's - so far I am not impressed at all.
When I would watch SD tv over my local network with the Solo's they would generally use up to 5.5mbps of bandwidth, with the Pro-HD's the same SD channel will use up 8mbps. This is using the s-vid input and not component, the player I am using is the Windows 2.0 program, not a browser plugin. I am quite surprised the Pro-HD's use more bandwidth even when streaming SD content. It really doesn't make any sense as the original source data from my satellite provider is no where near 8mbps. So while the picture really doesn't seem any better(its still VGA) its putting a lot more load on my PC. This is by no means a show stopper, just an observation that to me is a mistake on Sling's part.
More concerning is that using the Pro-HD's I can no longer use the HQ mode via 3G on my iphone, the picture cuts out every few seconds. With the Slingbox Solo I could use HQ mode for an hour straight via 3G and it may have cutout a few times in the hour. Now I know my DSL upload is a limiting factor here at only 625kps but why does the Pro-HD clearly need more bandwidth then the Solo to stream the same content?
I got a great deal on the Pro-HD's but at this point they are probably both getting returned as they have lowered my viewing experince on both the PC and my iphone.
To summerize - unless I am trying to stream HD content with the Pro-HD I really think the Solo and Pro-HD should use the same bandwidth. It also explains why Sling customers seem to have varying success with the various hardware/software combinations.