I have a 4G modem with a PAYG Three SIM as a backup to our DSL line. As the DSL has been extremely reliable, it hasn't seen a lot of use - mostly just as an out-of-band way to access the A&A status website and IRC channel. Throughput seems adequate (compares very favourably to ADSL2+, not quite as good as VDSL), latency is poor (currently averaging 56ms) with occasional spikes at busy times. Cellular network performance is a very local phenomenon, thobut, so YMMV (ours depends chiefly on the migratory habits of several thousand data-hungry students).
NAT issues can be worked around by tunnelling with a VPN (A&A provide an L2TP service for the purpose), but that's another layer of complexity and point of failure. Not ideal for VOIP.
(Three's name made sense back in 2003, when Hutchinson needed a brand name for their Shiny! New! 3G network. Predictably, it hasn't aged well. The last network with a sensible name was Cellnet, which turned into O2 at around the same time.)