I was testing Smarty as I was planing to move networks and noticed my phone was showing 5G (5G NSA according to Network Signal Guru, not "proper" 5G). They enabled it a few days ago.
It's not as fast for me as other speed tests shared here, but my flat is half under the street level and it's still faster than my wired ~60Mbps FTTC connection.
Indoors, by the window, I can't get past ~130Mbps during the day. I've seen speeds just over 200Mbps after mid night. If I open the door and walk just 5 steps, I can get very good
download speeds:
This is Vodafone 4G in the same spot:
Speeds can fluctuate a lot, especially during the day. One moment I might be getting 300Mbps, see a drop to 50Mbps for 30 seconds, and then an increase to 100 or 400Mbps.
I wanted to see how good or bad it was in practice, so for the past few days I've been using my phone's hotspot. Web browsing, video calls, 1440p YouTube videos, some heavy downloads (1-5GB), email, etc. After 100GB I can say that it's usable, but not super reliable/stable (keep in mind that Three's signal isn't good inside my flat).
While it works fine 95% of the time, sometimes I may have to reload a page because it failed to load. Pings can be as low as 15ms and then jump to 100ms. YouTube videos at high resolutions will be fine most of the time, but I may experience buffering during one of the speed drops. The slow upload speeds cause problems with VPNs (due to the encryption overhead) and obviously 5Mbps might not be enough if you have multiple people uploading stuff (eg: video calls).
Most of these issues seem to be related to Three's signal in this area... I'd like to compare Smarty's 5G to Vodafone and GiffGaff (O2) as I have their SIMs too, but neither support 5G in this area. In any case, indoors, Smarty's 5G is better than Vodafone's (40/10Mbps) and GiffGaff (terrible, 2/2Mbps) 4G. I think EE's 4G could compete with Smarty as I've tested with a PlusNet Mobile SIM and was getting 110/40Mbps during the day (and PlusNet has a speed cap and doesn't have access to all EE's bands).
In any case, even with the issues I have, I think this is a good alternative for many. If it wasn't the slow upload speeds and my need for low latency (I play online games), I'd cancel my FTTC connection. Not only download speeds are better (and the lower speeds sometimes are faster than FTTC), but it's also cheaper than FTTC: £20/mo unlimited monthly Smarty plan vs ~£30/mo.
I don't know if I'd do any long term contracts though. Three is investing a lot in 5G, but their cheaper prices paired with a lot of/unlimited data will probably lead to the same problems their 4G has in parts of the country. They oversold it and their speeds are a joke in places like central London. Unless they install one of those high band cells just outside, I'd be surprised if a year from now speeds are as good as they are today.