Fenders are a must if you intend not to let a little shower stop you from riding.

• Plastic fenders from Axiom and SKS are quick and easy to install, with much of the hardware already on the fender. They look OK and are pretty durable but not the lightest. They can develop rattles over time but will still work fine. Typically not full-length around-the-wheel protection.
• Aluminium fenders from Velo Orange: slow to install but not difficult. Comes with a few different options for hardware to install them to your frame. Fairly good length for more protection. Not that light especially the chunky hardware. Comes in a variety of shapes and sizes, and in black or silver.
• Aluminium fenders from Honjo: Lightest fenders. High-quality aluminium alloy with extremely refined shaping and finishing, made entirely in Tokyo. Very tasteful in smooth, very Too Much in hammered, both fancy-looking. Trickiest to install but most adaptable for custom fitting, since they aren't pre-drilled. Come with very nice hardware, and offer full-length protection. Many, many shapes and finishes.

Other than plastic fenders and clip-ons, all fenders require more or less custom fitting. Keeping the line fair around the wheel will cause less stress to accumulate, which could cause early failure.

All fenders benefit from adding a little flap to the bottom, they do a lot to keep water from spraying up at the ends of the fenders.