1) Depends *entirely* on what you're looking to use it for. They're good for stuff like Office (though you'll want the keyboard) and web browsing (full browser, including Flashplayer and dev tools!), plus most of the common tablet-y stuff (Netflix, ebooks, etc.). They are modestly hackable (you can "get root" trivially - just a UAC prompt - but you can't modify the kernel or bootloader in any meaningful way at this time). The app store is a lot smaller than for Android, so more esoteric stuff is likely to be missing.
2) No. (Really?? It's an ARM processor, not x86; this should be obvious...). Search before posting!
3) Other than Microsoft-signed ones, not until a jailbreak for RT 8.1 comes out. No idea when that will be, though it's in development. Even then, only ARM or .NET binaries will work (there's a project to support x86 ones through dynamic retranslation, but it was never completed).
4) Go away, filthy pirate; we don't like your kind here. Read the bloody forum rules before you come back, too. $DEITY, 6 years on this forum and you don't know that yet? This is a *developer* forum, a lot of us publish stuff on the store... I guess I shouldn't expect better of somebody with a less than 1 in 200 ratio of posts to thanks, though. (SIX YEARS and 600 posts and you don't know that??)