Thought I'd update to mention that the Kdenlive I'm running now (0.8) is far more stable. Certainly not perfect, or perfectly stable, but massively improved. Wouldn't recommend it to the world just yet, but for people with tolerance for the bleeding edge, it's not so bad.
I'm editing 1080p video. It (mostly) holds up. The 0.8 designation is reasonably accurate. I'm running low production weekly shows off this software (but they're 1080p... ...I understand the contrast) and it works. With the 0.8 series it works well enough. I was about to kick Kdenlive to the curb with the 0.7 series, and 0.8 came out.
Also, I edited more video with 0.7 from the time of the parent comment, and the timeline bugs with Kdenlive actually ate at me more than my cheap upload speeds and even the real stability issues. Those timeline problems appear to have disappeared 0.8. I haven't seen any.