Coming from an environment where we just put everything on shelves in racks, I have to say that proper rack mounting is kind of neat, but rack rails irritate me. It's all because of one simple little problem:

Why, oh why, can't people agree on a single sort of rails?

(Or at least a single sort of rails for things of each rack unit, so you'd have 1U rails and 2U rails and 3U rails and so on.)

We have server hardware from a number of vendors, and several generations of hardware from the vendor we usually wind up buying from (their name starts with a D). This has created an amazing assortment of rails and rail mountings, where it sometimes feels like no two servers are the same. And every so often we get to play the fun game of 'hunt the rails kit for this server (if we have one)'.

Of course, the rails and the servers are generally not labeled so that you can tell which goes with which; you are apparently supposed to remember what each different sort of rails and rail mountings looks like. This becomes important when you have loose rails and servers floating around, or merely when you have to relocate an assortment of servers (hopefully complete with their rails) from rack to rack and not lose track of what fits what.

(If you have rails kits sitting around, I strongly recommend getting a marker and putting a big label on their boxes about what sort of servers they fit. Now. Don't delay; you can thank me later.)

On a side note, putting numbered marks (in our case, marking off rack units) on rack posts has to be one of those great ideas (no more exciting 'is this side really lined up with that side?' games, especially when several people are working at once). However, I must say grumpy things about the kind of quality control in rack posts that results in screw holes that are just different enough diameters that the same screws go in easily in one rack post and must be forced hard in another. In the same rack.

Sidebar: a terminology note

'Rack posts' are the vertical strips that equipment is bolted to. The Wikipedia racks entry currently says that they are sometimes also called 'rails'; ignore that, because it will only confuse you. Rails are purely the too many different sorts of horizontal things that bolt into racks and support servers, possibly letting them slide easily in and out.