Black Templars' elite troops don't differ much from regular Space Marine elites, and their use isn't much different either.

Terminator Squad and Terminator Assault Squad

Terminators are very durable due to their excellent armour and their invulnerable save. They also look great on the battle field, so why not take them? For once they cost a lot of points. The other thing is that they can only use land raiders for transports. As 4th ed will make teleporting much more common, this might be less of a concern.

For Black Templars especially the terminator assault squad is an interesting option. A unit of seven with 4 pairs of lightning claws, 2 thunder hammers and a sergeant with power weapon can dish out a lot of damage. They can ride in a land raider crusader and there's still room for the emperor's champion to join them. Sounds like a deal, even though a very expensive one. These guys can rip apart enemies in close combat in an instant, on the other hand it's a big gamble: One lucky shot and you've lost a very expensive unit plus a land raider. With 4th ed transport rules it might become extremely difficult to get these guys into close combat once they've lost their transport, making them a waste of points. If you like them, take them, especially in larger games, but be aware that you are putting a lot of eggs in one basket.

Dreadnought

I must admit that I'm biased: I simply love dreadnoughts. They look great, they are very versatile and can deal out a lot of damage both in ranged and close combat. But I must admit that their armour can be pierced easily, so don't expect them to live very long, unless you bring enough vehicles to make it difficult for your opponent to take them on all at once. My favourite combination are plasma cannon and close combat weapon, but taking a lascannon instead of the plasma cannon is equally versatile. If you take two dreadnoughts, I'd take one of each. If you take a third, give one a missile launcher instead of the close combat weapon together with a lascannon. And always give your dreads extra armour.

Veteran Squad

While the ability to take close combat weapons instead of bolters might be an interesting choice for regular Space Marines, it is by no means special with Black Templars. Combine this with the fact that Black Templars' Veteran Squads can't have a veteran sergeant (= no power weapon or power fist and no terminator honours' upgrade), and you get a totally useless unit. I never use it.