See this thread for some responses from those that have actually tried them.
http://singletrackworld.com/forum/topic/who-has-actually-tried-a-29er-then/page/2
As I said ‘Don’t knock em till you’ve tried them though but unfortunately this is often the way in the UK.
My reply
I have a GF Paragon (with a 120mm fork / short stem) and a Trek (GF) Rumblefish and enjoy them both. I still have a nice collection of 26er bikes as well though.
29ers are great for blasting over rock gardens, roots, etc
Technical climbs are easier as they have more grip and roll over those steps that would stall a 26″ wheel.
The 29 hardtail gives a much more comfortable ride than a 26, probably mainly due to the bigger tyres and lower tyre pressures you can use.
Downhill confidence is greater as they feel very stable and you sit ‘in’ the bike between the wheels so it’s more difficult to go over the front.
Most of the time you feel you’re ‘cheating’ as everything seems easier and you ride faster.
Downsides are few. I guess they do take a bit longer to get up to speed but I’m running them both tubeless which helps. On really tight singletrack they feel a bit unwieldy and less responsive. It’s more difficult to get the wheels out of narrow ruts or gaps between rocks, etc.
Look out for them on ebay as they can be very cheap since the UK has little interest in them at the moment. I paid about £500 for the Paragon so I figured if I didn’t like it I’d probably get my money back anyway.
Don’t knock em till you’ve tried them though but unfortunately this is often the way in the UK.