Been using tmobile phone hotspot for a couple of years as my primary net access most of the year. It worked great until this spring. I've been having issues staying connected, getting gradually worse. Tech support is working with me on it. They are now thinking it's my phone, so they may give me a free replacement.
Tmobile needs an att card in the deep boonies / national forests, but their fantastic support staff make them very worth the minor inconvenience, IMO.
Are you thinking of getting them, or having troubles with a phone you already use?