I use one and am 6ft and its fine for me. At 6'4 though somethings his going to be brushing against the tent - either the foot box of your bag or your head. I think it will work for you but it will be snug.
I am 6'5" and had a copper spur UL1. I generally am a side sleeper so I am a bit curled up but I was able to fit. It was a tight fit though. I gave that tent away to a family member and now I just use a 10'x10' tarp which is way lighter and more versatile.
I am 6'5 and owned a Fly Creek UL2 for a very short period of time. It was a nice tent but way to small for me. Now I have a mega tarp, it's a mansion when comparing the two.
I have a copper spur UL2 and pitched it inside the house and slept for a couple nights inside the house to try it. I packed it up and ordered a sawtooth and haven't looked back.
My copper spur sits inside the closet and does nothing.
I'm 6'4 found it alittle tight. Returned it and got tarptent double rainbow, great lenght for a tall person and enough room to keep your gear in with you