I'm on my second bike with a lefty and really like them. Not done a huge mileage on my current one but feels good so far. Smooth, stiff, light with really effective hydraulic lockout.
Downsides are as OCP stated, you can't really work on them yourself, which means a £150 service. Standard Cannondale quirks are that for a lot of it you're stuck with Cannondale replacements, with little choice. That's fine if you buy into the Cannondale ethos, but for some people it's not for them. Velo put it best when he said that Cannondale is the bike equivalent of Apple, you either get it or you don't.
I had one of the alloy Flash 29ers for 3 years and it as a fanatic bike, but I do wonder if it's really suitable for what you've mentioned your needs are. If you like the Cannondale thing, have a look at their urban Bad Boy range. They even do ones with a rigid lefty!
