As others have said, how useful you'll find it will depend on your expectations pre-purchase. This will not replace your iPhone. It is not a toy you will spend hours playing with daily. It is, however, a more convenient way to get notifications, directions, activity tracking, and weather updates. If that sounds like something you'd appreciate, I think you'll find the current version of the Watch very useful. If this isn't what you're hoping for from it, you might be better to wait and see what the next version brings.
In general, though? My advice on this one is buy now if you can easily afford to do so.
Considering it's a new product with no established upgrade cycle, we can't be sure when the next gen will come out, so it could be a frustrating wait. Unless you really can wait for the next gen without knowing when it will come (which could be a year from now, or two-- potentially longer), the longer you hold off, the less time you'll have it before it's usurped by a newer version, and many people find that frustrating.
If you are not sure how much you'll use it, go for the Sport. At the end of the fortnight, you can swap for the SS version if you are loving it, stick with the Sport if you are liking it, or return if it's not for you.