How popular are alternative clients? Is there any data available on how many people use an alternative client in contrast to the Satoshi client? Alternatively, is there any data on how many times such clients have been downloaded?
Multibit download stats

MultiBit is a thin-client based on the BitCoinJ library. It is written in Java and comes with installer packages for Mac, Windows and Linux. It is intended to be used by the general public who don't have a great deal of knowledge about Bitcoin and just want to send and receive payments in the simplest manner possible.