I bought the 28/1.9 first. When I had – or, more accurately imagined I had – some more cash to spare, I bought the little 25/4 in Leica M mount. Since then, the 25/4 is the instrument of choice: it weighs nothing, is less quicker and demanding to focus on the R-D1's so-so rangefinder and you can use the whole finder to approximate the image coverage. Actually, this is a good idea for specs-wearers, because you can dial up the 50mm framelines and use them for accurate centering and horiz/vertical alignment, something that's not always possible with the hard-to-see 28mm frames.
Vignetting is a problem when you want even illumination and has its own charm when you don't (which I find is surprisingly often). One advantage the 28/1.9 is justly famous for is its naturalistic tonal rendering, when used on the R-D1: the 24/4 is far more contrasty – but I like contrasty images, so it's not a big issue, at least for me.
I've got to the point that the 28/1.9 is seeing so little use that I might well sell it on.
