The nydus canal is a structure that you should always use, especially if you have island bases. This is probably the best structure that the hive has to offer. The new Brood War ladder maps all have long expansions, especially 'Forsaken Wastes'. Now even if you don't use it for transporting drones, it's a great to move hydralisks and zerglings. Though whenever you plan to attack a Zerg base that has nydus canals, you should try to take these structures out as soon as possible, since they allow huge amounts of units to transfer from one place to another.
Now everyone you knows that you should use these structures for transporting drones once a base is used up. They make great structures for those high plateaus with no entrance. Now placing a nydus canal is more important that it may seem. You always want to place them away from your opponents. If you were to be attacked, your opponent wouldn't be able to destroy the nydus canal that's in the back, which can supply a huge amounts of hydralisks. You should also place all your nydus canals in one base, so that your links won't be totally broken if you lost an expansion, as compared to your main base.
Now to use a nydus canal for offense can be done as well. The best way to attack someone with a nydus canal, is to get a queen and parasite potential bases. Hitting the main base, might be hard, since most of the hatcheries are there, meaning most of the units get produced there. Try to find a nice undefended part of the base first, and attack his front door, with your forces. At the same time, create a nydus canal at his base.
Try to delay the battle long enough, by retreating and using hit and run tactics. It does take awhile before the nydus canal finshes. Once done, run in and raze the base. Another way is to build like 5 nydus canals, and then build them all simultaneously at the base you want to hit. Hopefully your opponent won't notice, but he probably will, but He won't be able to destroy all of them.
To defend against this, it's good to have a a few units scattered over your bases, especially ranged units. You should only do this, at the end, and when you expect it coming. You should prepare to defend against this tactic, if you
1) Rally point your forces to the front line,
2) Your opponent uses queens and hydralisks, or
3) You are playing on an island map.
Another powerful way to use this is to purposely build a hatchery, just for the creep. You can try this on any large map, or large island. It's a great way to move units, which may be detected if they walked. I used this once on Dire Straits, where I hid on the edge of and built a hatchery on my opponents island. Now once that was done, using a pre-built nydus canal, I quickly made a canal to the island. The rest was easy to figure out. Once I got my nydus canal, an endless stream of hydralisks kept coming out, making it impossible to destroy the canal.
Any map that is extremely large as Green Valleys or Killing Fields. Is just asking for a nydus canal. This is the same with any island map, or a map where half of the land has high plateaus.
