Arjac Rockfist, I thought I'd talk about his wolf riding brethren the thunderwolf cavalry. I've owned some third party thunderwolves for quite a while now and they've been a regular feature in my Space Wolves lists at 1,750pts and above.
Their mere presence is enough to strike fear into the heart of your opponent. Most players will know what they're capable of and will focus a lot of their efforts in to dealing with them. Here's how I run mine:
Thunderlord w/ wolf claw, storm shield, runic armour, sage of the bear
4 Thunderwolf cavalry (1x storm shield, 1x power fist & 1x melta bombs)
At a total of 510pts it's a serious chunk of your army. However, if things go according to plan they can easily pull their weight and then some. There are very few units out there that can deal with a full squad of TWC charging them. Even with the ability to insta-kill them grey knights will struggle to score enough wounds (even with hammerhand) to severely cripple them. Keep them fairly close to a rune priest and you might even cancel their force weapon anyway. Very little else can deal wtih them effectively even Mephiston won't like the sheer number of high strength, rending hits coming his way.
That being said I have had games where they're severely underperformed. Despite having a massive 12" charge range they take their time getting there. A few bad "Run" rolls and they might not be in the action until turn 3 or even later. In practice though, I find opponents tend to misjudge just how far they can cover with their 6" move, D6" fleet and 12" charge. In the games where they've done badly this has been down to them being thinned out by long range fire earlier on in the game. However, with a pair of storm shields and the lord's 2+ save it takes a lot of hammering to weaken them. This is all firepower that isn't hitting your rhinos or the grey hunters inside.
If you don't want to commit so many points to a single unit some people run a thunderlord solo. Although it's tempting to give him a couple of fenrisian wolves as ablative wounds, remember they will make the average toughness of the solo unit lower and hence make him easier to wound. There's a lot to be said for Saga of the Warrior Born but personally I find there's too much instant death around to risk not taking Saga of the Bear for the same points. It's sometimes sickening how long a wolf lord survives even after his packmates have got to face Morkai.
To be perfectly honest I don't care how well they perform. For me they're the best unit in the codex. Yes, they don't always perform but they're great fun to use and are a lot more characterful than terminators in land raiders which other codexes often do better! Think of it this way, for the same price as that land raider full of terminators you can have your sons of Russ riding around on dirty great lupine monstrosities!
If you've been toying with the idea of getting some thunderwolves I strongly suggest you bite the bullet and try them out.