I have to admit, no one really uses spider mines only because siege tanks are better. Spider mines are only suited for a few purposes and really are made to be used against the Protoss. However spider mines are useful. Who wouldn't want to have 3 mines and a vulture for 75 minerals. This is one of the best deals in starcraft, except for maybe scourges and zerglings. A vulture by itslef is a good support unit for taking out small units as zerglings, infantry, zealots, and of course workers. Having a range attack, it can take out a worker line pretty fast, making for a good raiding unit.
Now spider mines must be upgraded before your vultures get them. Of course once the upgrade is complete, all of your vultures will be able to drop 3 mines. There isn't anyway to reload mines, but if you make a new vulture you'll get your 3 new mines. Now mines, once placed, are invisible to other players who don't have a detectore nearby.
Spider mines also will attack any land unit that walks close enough, cloaked or uncloaked. So dark templars and ghosts aren't a good idea for walking through a mine field. Some units won't trigger a spider mine. These units are all the basic workers of each race, any vulture, archons and dark archons. Making every other unit targets for the mine. Now a mine will do 125 damage splash damage. This great for breaking up any attack.
Placing mines is another skill. Dropping them in narrow choke points is obvious. However you should make sure that you use them efficiently since mines can destroy each other.
Now using them to prevent reaver, hydra, tank, and templar drops is another use. Place mines around the outskirts of your base, around where the units will most likely drop, since your turrets don't cover every area. This will prevent any drop, and give you better sight on your minimap, for any upcoming attacks.
Spidermines make good scouts, since workers won't set them off. You can easily find any expansion that your opponent plans to build by placing one mine at each mineral site. And since mines don't go off when a worker approaches, this can warn you if someone is trying to cannon, or bunker your base.
Another trick is place a huge mine field at a key point where the enemy will gather there forces. Most likely outside your base, or at a choke point outside there base. Place all your mines and get a good spread. Now turn ally on, and wait. Unless they attack from the air, which is unlikely on a land map such as hunters, and given they know that you have turrets setup, you can just sit back with trap. This is risky since some players won't setup outside your base, but then it's even better since your mines will hit the bulk of there army as they march in a line towards your base. You should easily see them coming since your mines give a decent sight of the area.
You can also blind your own spidermines, making them explode as a unit walks directly on top of them. Very effective if your opponent is using a lot of range units. Though it doesn't help to scout the area since they are reduced to 1 sight, but killing hordes of units is well worth it.
