This is actually my first smartphone, so my perspective may be somewhat different from that of the reviews of it out there. I am a software engineer, and I greatly value the freedom to do whatever I want with computing devices I own, and to be able to tweak things and contribute any enhancements back to the community.
If something irks me and I can fix it, I will submit a patch (i.e., as a drive-by contributor), otherwise I will file a bug report and help reproduce the issue. Nothing major, no huge time-sink; just being able to help shape the software that surrounds us, and having a feeling of contributing to something bigger than a single corporation. So naturally I use a flavour of GNU/Linux on my computer.
I already use Ubuntu on my desktop and laptop computers (both professionally and private), so the Ubuntu smartphone seems like a good shot at getting something that can potentially near the freedom I have on desktop Linux, and thus far I am not disappointed.
A smartphone for me is a way to get information on-the-go. For things like public transport itineraries, weather forecasts, and navigation Ubuntu Touch already suffices.
The big 'issue' of course is apps. Ubuntu Touch has its own store with a growing number of free software apps, and some proprietary stuff. Most of the basic apps I expect are there, mostly pre-installed. Big name apps are missing though — there is no WhatsApp for instance. Now for my opinion on this topic, which may very well be an unpopular minority view:
To me, this lack of big name proprietary apps is just fine and exactly what I want. The term 'app' is ambiguous at best and I feel that it is a threat to software freedom. To illustrate: A simple stand-alone off-line calculator is called an 'app', but so i a web-service accessible exclusively with proprietary software like WhatsApp. That is, an 'app' in common parlance is not just the piece of software you install on your device, it is also the infrastructure backing it. In the case of the extremely popular WhatsApp, you need their software to be able to access their service. It appears to be illegal to create a free software client that can talk to their service, unlike, say, email (SMTP, IMAP, etc.). That's fine, its their service, their rules, I prefer to make due without.
I don't want a computing device filled with a bunch of proprietary clients for day-to-day tasks. If you offer a great service, offer a great web-app to use it with (I already have a capable browser installed), or (re)use or create an open protocol anyone can use. Otherwise, I'll just do without. If your business model depends on me installing a piece of proprietary software, I am not interested.
Other than that, there is still plenty of rough edges. It is not trivial (though possible) to install the normal software packages from Ubuntu's repositories, but on the other hand, I have not had the need yet. I do sorely miss Firefox with uBlock Origin though! Ads on websites seem even more intrusive than on the desktop.
If I had to summarize the threats to Ubuntu Touch's future I would say that it sometimes feels as if there are not enough core developers working on the product, but that may be just the impression I got from the issues I have reported.
Canonical seems to bet on being able to get Ubuntu Touch entrenched in developing countries, but I doubt that that strategy will pay off (although I am not knowledgeable enough on that topic to make a firm statement about this). If they can manage to make Ubuntu Touch synonymous with the easy to use and obtain free operating system for smartphones (the way Ubuntu is for desktop computers) they may have a good chance at winning the hearts of software developers with a preference for free software — that is the niche I would bet on.
The lack of apps in the way of Android and IOS is of course problematic in the current app-economy driven smartphone world, and will prevent major adoption of Ubuntu Touch for some time to come, but it may be that I am not alone in my dismissal of this reliance on proprietary apps.