My M8 experience is mostly like Jono's, with one small difference. I have coded all my 35mm and 28mm lenses. My 50s and 90s are not coded. I leave the UV/IR setting in the menus all the time. Works fine.
Coding the 28mm and wider lenses is a must. Coding the 35mm is a good idea--you may not notice the slightly cooler corners under most conditions, but you will be better off with coding. With most 50s, it really doesn't matter on the M8, and does matter on the M9. With the Noctilux, it probably does matter, because that lens vignettes a lot, and dealing with both brightness vignetting and slight cyan shift in the corners is complex.
There is some anecdotal evidence that if you send your lenses to Leica for coding, they might come back with the focus somewhat off. They take your old mount of and give you a new one, and sometimes have not adequately readjusted the lenses to digital M tolerances. I don't know if this is a thing of the past, or if it still happens. I always use DAG for my lens adjustments. He adjusts your lenses to a known "good" M8.
You should always use a UV/IR filter on your lenses, coded or not. Otherwise, you get magenta synthetic black fabrics, overly ruddy skin in tungsten light, and yellowish foliage in sunlight. You also get less sharp pictures, as each picture has a slightly unsharp IR component overlaying the sharper visual light component. Some people like that look in B&W, or like to play endless games with color correction. I don't.
I use the M-coder kit for some lenses that I didn't want to send away. It works fine. If there is a screw in the coding area, you can cover it with white nail polish, then add a black line if necessary. Some M-coder kits are slightly off in alignment. But once you discover which direction it is "off," you can simply thicken the black marks in the opposite direction, and all's well.
Hope this is useful!
--Peter