I have faced this issues couple times on different Ubuntu versions while trying to install MariaDB (fully compatible MySQL replacement DB)

dpkg: error processing /var/cache/apt/archives/libmysqlclient18_5.5.33a+maria-1~raring_amd64.deb (--unpack):
 libmysqlclient18:amd64 5.5.33a+maria-1~raring (Multi-Arch: no) is not co-installable with libmysqlclient18 which has multiple installed instances

as I understand "old" version of libmysqlclient18 is already installed and for some reason system can't resolve what to do with current lib as some other soft depends on it.

Fix

Open /var/lib/dpkg/status search for a line with Package: libmysqlclient18 and remove block of text till lines "Original-Maintainer: ..." this text block could appear couple times there, remove all appearances. after that run apt-get -f install

Such "hack" helped with some other libs also, whenyou are sure that you have installed correct version of library or you have a PPA with needed version, but apt-get does not see it. Try do the same manual cache clean for this lib.

Try LiveCD before update

Before making an update to new version it could be a good approach to download an ISO, to make a LiveCD and try that new distribution works in LiveCD mode well on your hardware.

If you already have installed Linux Mint 15 and want just to update your current installation to latest version do the following (but always backup any critical data even before proven update operations).

Replace distro codename in sources.list files raring => saucy and olivia => petra

$ sudo sed -i 's/raring/saucy/' /etc/apt/sources.list
$ sudo sed -i 's/olivia/petra/' /etc/apt/sources.list
$ sudo sed -i 's/raring/saucy/' /etc/apt/sources.list.d/official-package-repositories.list
$ sudo sed -i 's/olivia/petra/' /etc/apt/sources.list.d/official-package-repositories.list

These are 2 required files, but if you had installed some PPA or repos review them manually in /etc/apt/sources.list.d folder and do the same.

Then run update

$ sudo apt-get update && sudo apt-get dist-upgrade
$ sudo apt-get upgrade

I am not Unity hater at all and really like the main idea and general look&feel of latest Ubuntu interface, I don't even care about these "privacy" flame around their "smart scopes".
Hey, I even use Ubuntu font in this blog :)

But the problems are in tiny aspects of usability (speaking about Ubuntu 13.10).

Sometimes after update or clean install of PhpStom on fresh OS you could get it with bold and hard to read font. Changing font in Apperance section could help. But the best solution appears to be removing fonts-unfonts-core package.

sudo apt-get remove fonts-unfonts-core

it is used for Korean true type and if you do not use this language, it is safe to remove

Recently Canonical anounced new icon theme for Desktop and Mobile by Matthieu James (author of famous Faenza/Faience icons sets) that was hired by them. It looks really nice and consistent, but while we are still waiting for 14.04 with this new set, let's review some alernative icons sets for Ubuntu.