I’ve used the Eagle Creek pack for about 5 years now, both on a 10 mile road commute and now on a ~45 minute offroad commute, and it works very well, after 15 mins wearing all noticable creases are gone (just don’t pack shirts into it straight after ironing them – then the creases get “set”).
I tried upgrading to a shirt shuttle and didn’t get on with it (more creases and it’s a lot more bulky) – must get around to selling it.
<edit> just saw the comment above about the shirt shuttle not being so good for larger shirts which ties up with what I found (6’1″).