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.