http://i.imgur.com/JFFzgiv.gif
I've used two Lenovo flagship phones, the K900 and Vibe Z (1). The breakdown:
Software: disastrous, atrocious, horrid, I can not stress their incompetence in this area enough. Their old Ideaphone UI was a nightmare, a car crash, a cataclysm, let's just move on. The Vibe UI they switched to later is a silly, childish and uninspired knockoff of IOS that erases everything good about android including your precious app drawer. It comes with a bullshit "security" app that hovers over you at every juncture reminding you it's watching. Oh and remember you're likely to be using a Chinese version with poorly adapted English as a secondary option only. I'm not joking, it IS this bad. Be prepared to root and mod extensively or you will hate these phones.
Hardware: a mixed bag; very good on the outside. The K900 was gorgeous, thin and light as a sheet of paper and had an excellent screen. The Vibe Z's weight was even more impressive, matching that of an average 5 inch phone despite being 5.5 inch, and it had decently small bezels. For some reason Lenovo only make good oversized phones, though. As for the inside...
Performance: inconsistent, a lottery. K900 ran like absolute poo on Western roms but suddenly started performing alright when I flashed the chinese Vibe Rom. Vibe Z was alright on most fronts, not stellar but as good as an S800 is expected to be. And...
Battery Life: not their strong suit. In the battle between thinner phones and longer lasting ones they're firmly on the former's side. Their optimization skills are also not on the level of Western OEMs' yet. K900's battery life was just terminally unacceptable at first and became OK after flashing the better rom. Vibe Z did about what you'd expect from a 5.5 incher on 3000 mAh. I suppose the good thing is it was light and thin despite having that fairly large battery. The biggest problem with their battery life philosophy is that it precludes their phones from being well usable as phone-tablet hybrids. Use them as a regular phone and they're OK, but why would you do that when your screen is 5.5 inch..?
Modding Community: mainly consists of Russians, Indians, Chinese and other creatures you can't expect to speak full English sentences to you, let alone take the time to write them out to explain technical issues surrounding modding and flashing to you. Another point of frustration.
Recommendation: if you have an appreciation for irony and a masochistic streak buying one can be something of an adventure. I don't regret the experience much myself but its not for everyone. Not at all. Some of their models are very cheap these days, in that sense they have a major saving grace. But if you want a comfortable smartphone experience this is a brand you should avoid like the plague.