La Vida Leica just posted a review of the ZM Biogon 25mm f/2.8.
This 25mm is reputed with its insane resolving power at 400lp/mm. It definitely out-resolves any of the (normal speed) film and digital sensor made to date. So if your friends are saying X megapixels is enough already, Y is way too much, then you can tell them there is still plenty of room for improvement before we hit the optical bottleneck.
Check out La Vida Leica's review here. Also available is Steve Huff's review for comparison.
You may also want to check out La Vida's other ZM reviews on the 50mm C-Sonnar and 35mm Biogon.