If I recall correctly iconbar (sorry Chris/Peter) had a poll which suggest 20% of respondents use RO5/Iyonix and 27% use Select (select the view previous polls option). They've also started a new poll (it must very new as my single vote changed all the stats !!!!!) which asks various questions about whether people want/don't/don't care etc., if Select is on the Iyonix or not.
Granted this may not be the most *scientific* way of finding out (for one thing its hard to tell how many people actually voted !), but it is at least some measure.