From my experience with zLinux environments I would be inclined to scrutinise any density claims very, very carefully.
Interesting they're working with Canonical on this - the Z has historically been a SuSE stronghold, with a lot of the development of Linux-on-Z happening in Germany, and Red Hat trailing behind. I'd be wary it will be another increasingly typical adware effort from them.
(I follow the main zLinux mailing lists for my day job, and I see a lot of folks from SuSE, some folks from Red Hat, a bunch of people from IBM, and basically no-one from the Ubuntu/Canonical world. Usual disclaimers apply.)
I can think of two good reasons. One is if they want to offer these for virtual desktop environments (as an alternative to the VMWare on Cisco UCS setups many current IBM mainframe customers run) The other to offer an alternative platform to EC2 where Ubuntu seems to be very common. Both would indicate that they are probably aiming at the desktop/user/development side with this product.
IBM's focus is on growing its alternative platforms - more users - not more tech. Tech is necessary but not sufficient. It makes sense to partner with Ubuntu if you want to focus on growing communities.
"typical adware" - heh heh! Seriously, in the consumer space everyone wants "free" so affiliate marketing is a sensible route - see the consumer internet for evidence. In the enterprise markets customers are focused on delivery outside the pure bits - open source bits are necessary but not sufficient - subscriptions such as Ubuntu Advantage with SLA's, management frameworks and consulting are viable there.
Ubuntu also inherits quite a bit of 's390x' platform compatibility work from Debian. For a user coming from a non-mainframe Linux background who wants the normal packages to "just work", it might be further along than other Linux distros (but: I have not quantified this). Debian-on-Z doesn't have as much testing as x86, of course, but since it's one of the five officially supported Debian architectures (x86, ARM, PPC, MIPS, S390x), it gets constant autobuilds of the whole archive, a reasonable amount of debugging effort, and release-engineering attention.
So I can only speak fairly generally, but I've never even met anyone running the kinds of ratios IBM is talking about in their press releases. I have, however, seen some workloads run pretty well; Oracle is IME a very good performer on Z, needing a smaller SGA and fewer IFLs than the equivalent Intel setup. Depending on the deal IBM offer you, zLinux can be a very good way of running Oracle.
Conversely, CPU intensive Java don't really seem to enjoy much, if any, advantage, which makes it a very, very expensive option for doing that.
There are a bunch of factors to consider (for example, if you've got zLinux and zOS co-resident on the same system there are some interesting things you can do with your legacy and modern code bases), but in general I'd want to prove anything myself before diving onto a Z.
Interesting they're working with Canonical on this - the Z has historically been a SuSE stronghold, with a lot of the development of Linux-on-Z happening in Germany, and Red Hat trailing behind. I'd be wary it will be another increasingly typical adware effort from them.
(I follow the main zLinux mailing lists for my day job, and I see a lot of folks from SuSE, some folks from Red Hat, a bunch of people from IBM, and basically no-one from the Ubuntu/Canonical world. Usual disclaimers apply.)