I'm totally convinced in the with the big wheels, far superior to a 26er in most/alot of cases. I cannot see me buying a new bike that doesn't have 29" wheels. The pricing and second hand maket sees me still buying 26ers but not for long. I see my next 2 moderns being 29ers anyway, replacing their 26" counterparts.
Could you convince someone else? What are the benefits? More wheel momentum, better roll over obstacles? The drawbacks? More inertia to overcome, heavier wheel/tyre combos? I have never ridden a 29er but am interested in the concept, though not completely convinced. Interestingly Geoff Apps was always a believer in 29" wheels but was mostly held back by lack of tyre availability.
It'd be hard to totally convince someone all I can say is read read and read some more then decide if you want to make the leap. You will find many people waxing lyrical about their 29ers and as many naysayers mostly saying they look odd. Unfortunately I don't think a test ride is often enough to really show you what they can do, whilst you don't have to learn to ride again there are things your mind will stop you trying because you can't do it on a 26er.
The benefits for me, are most noticeably the roll of the big wheel. I have a rigid and singlespeed which I reckon highlights this the most, its almost like a suspended bike but maintains the accuracy of a rigid. Small bumps are dispatched easily and your flow is unaffected by trail obstacles that would slow down the smaller wheel. The increase in carcass size makes for a very grippy and comfortable ride but not at the expense of speed. The bikes also tend to sit you between the wheels, in the bike as it were rather than on top, which also also makes for an entertaining ride. I've found I can get up steeper hills and down more technical obstacles, tackle trails at a higher average speed and its the bike I generally pick out first nowadays.
Downsides? Well the bike I have is a fond of staying on terrra firma, even mild drop offs are a bit of a pain. I will attribute this to the bike though. New models are coming out with geometry to tackle this if you wish. Price is still high as EoB mentions, second hand stuff is coming through and on one have been championing them for a while and prices are pretty fair.
The myths about slow turning, only for tall folks hard to accelerate are all slowly being proven wrong. You can now get a 29er to do more or less what you want it to.