Just how good is the 20/1.7?

I have the 20 and consciously chose it over the 25, although I could easily afford either one. But that's me and I can certainly see why others might think differently. The pros and cons are in my opinion as follows:

Pros for the 20

• Smaller
• Lighter
• Less expensive
• As sharp or sharper at really wide apertures
• At least as good bokeh if by bokeh you mean the quality rather than the quantity of the blur
• Wider FoV makes it more useful in close quarters
• Reportedly doesn't cause as much aperture clicking ("rattle-snaking")

Pros for the 25

• Better low-light ability (about half a stop)
• Can provide more background blur by means of its longer FL and greater max aperture
• More narrow FoV gives it more reach and makes it possible to take images at close distance/high magnification without too much distortion
• Noisefree AF (may be important for video where the noise is recorded if you AF during the clip; not otherwise)
• Suitable for AF-C (the 20 is not)
• Is not reported to cause lock-ups with the E-M5 (the 20 is, although I have never experienced this myself)
• Is not reported to cause banding with the E-M5 (the 20 is, and I have seen it myself; but see further info below)

There has been much talk about the slow AF of the 20. I have tested that myself and found only a very marginal difference in ordinary AF-S compared to lenses that are known to be at least as fast (perhaps faster) as the 25. See here:

http://forums.dpreview.com/forums/post/50110471

As to banding when the 20 is used on the E-M5, see here:

http://forums.dpreview.com/forums/thread/3285963