Many quality shops run a demo day regularly - they take a heap of bikes out to a trail area and let you test ride them which is far better than a spin in the carpark.
You'll find many XL frames have a differetn geometry than the smaller frames, many makers do this. On most 29ers they do run as short a chainstay as possible to maintain rear end stability, rigidity and climbing ability.
Remember, a 29er is basically a 700c wheel (622mm bead seat diameter) with a wider rim to allow for a fat tyre, so to maintain performance you need to have reasonable close tolerances of tyre/seattube.
Mavic make an excellent 29er crossmax wheelset, I have a set on my Audax bike and they boast many of the strengths found in the 26 versions, however if you're a clydesdale then a dh sun ringle wheelset might be a better bet, not sure if the deemax is available yet in 29.
Haha, yeah some people are hung up on the religion of rigid SS with their 29ers, calling themselves purists

but 29ers do make sense. Some say they are a bit slower to turn, but these riders could also be using super wide dh bars that are naturally slower. Slightly higher COG, really have to analyse the frame geo to see how low the BB drop is. Also check to see the length of the standard crank, would want at least a 180mm if you were looking at an XL frameset.
Robatman, did the shop you bought from take the time to measure you up and fit the bike properly? This can make a world of difference, sometimes a change of stem/seatpost, post height and seat position can be all that's needed.
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