A guy on here, Barry, rode with us at Mahlon a few weeks ago and rocks a Van Dessel 29er HT with a lefty, all pimped out with XTR, Stans, magura, hope rotors, etc. 23 lbs. That was one of the cooler 29ers I've seen.
I had an SS IF Ti built with a 1.5 headtube for the lefty and absolutly love it...however getting it serviced sometimes is a pain but I can send it directly to Cannondale.
Also the newer lefty without using an adaptor for bike with 1 1/8 heatubes has and integrated stem that slides down the whole headtube to clamp the fork...it's a much lighter setup and for the XL+ custom ride I put together it's under 23 lbs. easily...maybe even less than that.