I am currently using my nexus 5 on Straighttalk (ATT sim). I absolutely love it. For my area, it has almost as good of coverage as verizon, and the LTE is on average 4x-7x as fast. My average on verizon was about 4mbps. I am averaging 18-30mbps on ATTs network. I don't know if it was my old phone making verizon so slow here or what. it was an OG droid razr maxx.
Talk, sms, mms, LTE, etc all work great. Just remember, data isn't *really* unlimited on straight talk. I guess actually it is, but after you use 2.5gb, they throttle you way down. Super slow. I am going to used it for a few months, if i end up going over the 2.5gbs much, i am considering trying T mobiles 70 dollar truly unlimited data plan.
Good luck and enjoy your nexus
PS, here is a speed test done on LTE about 5 minutes ago.