Unity is all about re-arranging the deck chairs on the titanic.

The relative failure of Linux as a traditional desktop platform has very little to do with the UI. Sure Gnome 2 looks clunky and outdated now but it's basically a Win2000/XP Clone as far as UI is concerned (startmenu + taskbar + quicklaunch). So it will have been familiar enough to most users who come from a windows background (who are going to be the ones most wanting to try it). The UI is the not big issue here...

The problems with the Linux desktop for "normal users" (whoever they are) are and always have been:

Lack of ports of popular commercial software for many tasks and in many (not all) cases a lack of a "good enough" open source alternative.

Lack of reliable support for many consumer hardware configurations that are bundled with cheap desktops (nvidia/ATI support still isn't 100% for example), also on some netbooks you install ubuntu and the Fn + F(Key) combinations don't work unless you know to install a specific package. Also support for niche hardware for some tasks is hit/miss.

Weird intermittent issues that some people experience with power saving , wireless , flash etc..

No amount of changing the dock/menubar will fix any of these issues.

Let's face it , default unity is ugly.. it makes windows 7 look gorgeous by comparison but this has pretty much always been the way with Gnome/KDE. For many users (like me) the customisation aspect has been more than enough to make up for this however, unity pretty much kills that.

The only reasons I can think to advise anybody to run a linux desktop are:

You really care about OSS ideals and will not use any non-free software (in which case you want debian not ubuntu).

You are super paranoid and want something secure to install in a VM for using online banking etc.

You want a second OS so that you can diagnose more easily whether something is a hardware/software problem.

You develop software that will run on a Linux server so want a desktop environment that is as close to production as possible (this is me and probably most serious workplace Linux users)

Your a geek and like playing with different OSes

If somebody genuinely only wants to run facebook/youtube etc then pretty much any OS out there will suit their needs, in which case they will want to move over to something closer to iOS / Android rather than some half baked unity.

Even Microsoft have acknologed that you can't really easily build a UI that will work for the casual tablet / netbook user and the "content creator"/business user hence the seperation of metro and the standard Windows UI. I have almost 0 faith in canonical succeeding here.

Building a super simple UI on top of Linux should be left to the likes of Google/HTC,
with commercial OSes being increasingly locked down a space is opening for a serious "power user" system with high customisability, this is where Ubuntu could win big.

Of course people will say to me "oh your not the target market , use another distro"
Well there are a few problems with this argument:

The reason I use Ubuntu is because it is the closest thing the Linux desktop has to a defacto standard setup. Anybody who cares about distributing Linux applications will make sure they work with ubuntu and usually provide a tested .deb or an apt repo. If I switch distro there is a fair chance I lose this and end up back with source tarballs and weird install scripts. I spent a large part of the last 10 years trying different distros and this has consistently been the worst part of the experience.

Just because I am a power user does not mean I don't want my applications to "just work" and be installable through a standard simple interface, Ubuntu does this very very well (for the most part).

I think the whole idea of having different distros for novices and advanced users pretty idiotic really, a good distro should install with sensible defaults that "just work" and allow anybody who wants to customise to a greater or lesser degree. Many developers and other advanced users seem to have no problem customising the "beginner friendly" mac OS for their needs.

I could go on , but I think I'll leave it there :)