HS doesn't seem to be entirely legal, I wouldn't be comfortable giving them any payment details, that's just me though. Big brother is watching
Putting that aside the pros are obvious, no blackouts, short-term subscriptions possible and other leagues available in addition to the NHL. What's a bit off-putting is the fact that they're not telling the truth on their info page where they're comparing HS to GC, not exactly serious business... on the other hand, GC wasn't completely honest about blackouts in Europe last season either.
HS's highest quality streams are definitely nowhere near as good as on GC (*a bit more on that later on), and the frame rate, evethough noticeably over 30 fps, isn't consistent.
As pointed out before you get a way better frame rate with GC using apps on external devices compared to watching it on the PC via the browser, some of the apps have been a disaster last season though, what's the point in having a crystal clear picture running at a smooth 60fps if the stream stops every minute to buffer for 20 seconds...
*The good news is there's a way to watch those shiny 4500 kbps / 60 fps streams on your PC as well,
using XBMC with the GC add-on. I'm no legal expert but since you need a legit account for it to work I assume it's at least not illegal.
The developer himself says he's not sure if the add-on is going to work this season

, but I've tried it on a few pre-season games and it seemed to run properly. Of course, it comes with a sacrifice, the add-on doesn't support GC's PVR function, which means you have to watch live games actually live, there's no way to pause, rewind or FF, which means if you're home late for the game your options are to either start watching while the game is already in progress or if you wanna watch from the beginning wait until it's over and archived.