I can probably answer point one.
I think supping is a pretty good core body workout that you can have without knowing you are doing it. By standing up you are higher than when sitting on a kayak and as a result you get a better perspective of your surroundings. The boards are multipurpose and relatively easy to get around with, which makes them a bonus. So I guess it depends as what you mean by "inefficient"
No, it's not going to travel as fast as a kayak. As for awkward - I think that may have more to do with your paddling technique. I find paddling upright and digging deep into the water with your paddle while standing up straight is very good for your back and posture. There is a whole thread about this topic here:
www.seabreeze.com.au/forums/topic.asp?TOPIC_ID=38392&SearchTerms=back,problems As for gruelling - you choose the pace.