For 7.99 Euro I can purchase Texpad in the Apple store. It is just 11.4 MB, so it won't be as complete as my TeX Live 2012. Has anyone experience, how complete it is? Is it just the standard classes or could I prepare my Beamer slides with it?
Texpad developers here.
Since version 1.1 Texpad for iOS has had a built in LaTeX typesetter, no internet connection required. Most commonly used LaTeX packages are now included, and we expand the distribution with every update (9 updates in the past three months), and it is now so good that one user edits LaTeX on his Mac and typesets on his iPhone because the distribution is leaner, simpler and faster.
PGF drivers are underway right now and the update with local TikZ/Beamer typesetting should be ready for download in Feburary.
In addition to the onboard typesetter, we still provide free cloud typesetting servers with the entirety of TexLive 2012 installed. If you have any further questions, email us at firstname.lastname@example.org.
EDIT: update since Texpad 1.5
In Texpad 1.5 we added PGF drivers, a virtual font system and a Bundle Manager (http://support.texpadapp.com/kb/typesetting-ios/latex-bundle-manager-on-texpad-ios). Now Texpad is able to typeset virtually any TeX font or package with its onboard typesetter. This includes, but is certainly not limited to, PGF based packages such as Tikz and Beamer.
NB the reason for the small download size is that bundle manager allows you to install only the necessary bundles when needed. If files are reported missing during a typeset, the relevant bundle will be suggested to you.