what is the most user-friendly desktop client?

[JompinDox]

I mean a client that doesn't thrash a normal user's hard disk / bandwidth when installing???

[Xenophon]

If you want to get started quickly and not download the whole blockchain, I'd recommend MultiBit. I also find its UI very user-friendly.

[Kazimir]

I'd recommend MultiBit

+1

Best client imho.

[BookLover]

You can get around the "hard drive/bandwidth thrashing" on the Bitcoin-QT client by downloading the block-chain manually and running Bitcoin with the -rescan and -checkblocks parameters the first time you run Bitcoin with the new block-chain files.

P.S. This is simply a faster more efficient way to do it.  Not the best way if you don't want to have your own copy of the block-chain.