Re: zRAM on Ubuntu 13.04 x86/x86_64.
In theory, zRam has the largest benefit on low-RAM systems with a CPU that can cope with the compression. zRam should always be faster than on-disk swapping for devices that have a suitable disk but the biggest gains should be for SSD-based mobile devices. These don't have swap capability (normally) so when they run out of memory bad things happen.
In my experience of zRam in Ubuntu, my 1749 has enough RAM that it's never (noticably) used. On my 311c the CPU can't cope and bogs down - this is despite the fact that 3GB should be enough to run the system normally (I'll have to try it again, though, as I've just replaced that multiple-upgrade with a clean quantal install).
On my Desire Z, zRam appears to make little difference. On the one hand, I can run more programs. On the other, it interferes with Android's memory management meaning more apps simply run more slowly as "stale" memory isn't reclaimed as often. It also has the effect of slowing the phone when in a low memory condition, rather than killing a background application. This can effect functionality such as receiving phone calls. Disclaimer: I run unofficial CM builds (currently CM10+) so YMMV.
Overall though, if done properly zRam should provide a benefit to most users without them noticing it. It should provide an extra safety zone for low memory conditions without requiring horrible disk churning. The benefits, though, will be limited to newer machines. Older ones are better off without it.
1) ASUS GL702ZC: Ryzen 1700, 32GB, RX580, MATE@Manjaro-x64
2) Lenovo X230: Core i5-3320M, 4GB, Intel HD4000, Intel 6205, MATE@Manjaro-x64
3) MSI GT70: i7-3650, 16GB, GTX680M, MATE@Manjaro-x64