Alright, as promised, here is the Demolisher tactica.
One of the two tanks to carry over from the old Codex, the Demolisher is really the "Kill it dead" tank of choice. It's universally effective against armour and infantry, and is pretty tough, having boosted Rear Armour so that those damn Boltguns can't glance it. It's downside is the range of it's main gun, 24in, which means you have to mix it up with the enemy if you want to kill things. It's also more Expensive then the basic Russ, but the upgrade in firepower makes the price tag worth it.
Let's talk about what you should be taking on your Demolisher.
Hull HB/ Lascannon: It really depends on what you kit the tank out for. If you want to pulp hordes of Nids, IG, and Orks, take the HB. If you're fighting againt MEQs, Nidzilla, vehicle heavy lists, etc, take a Lascannon. The Heavy Flamer option is worth mentioning here, as it can come in handy if you want to use your Demo to Tank Shock infantry or bash apart Gunlines.
Sponsons: Ah, now here we go. Sponsons are a must have for a Demo. HF Sponsons can be useful if you have to burn through hords of enemies, but I would recommend HBs instead, especially coupled with the Hull HB. For heavy targets, I would say take Multimeltas, combined with the Hull Las. That combo will slag anything you point it at. Plasma would be above Multimeltas if it wasn't so damn expensive. If you can spare the points against MEQs, take the Plasma Sponsons, but otherwise, leave them at home. The one down side here is that if you move the Demo, you can only fire one weapon in addition to the Cannon, so plan carefully.
Pintle Mounts: I would save the points for better things, but there isn't much harm in taking one. Only combine it with the HB Demo though.
HK Missile: I will never recommend these on an IG tank, to be honest.
Dozer Blade: Yes! The Demolisher MUST move to be effective with the main gun, so having a Dozer blade only helps to keep it going.
Extra Armour: A decent choice, but not an essential one. The ability to move when stunned helps out the Demolisher more then any other Russ, but it isn't too critical, considering that the points can be better used elsewhere.
Camo Netting: Pass on this one. As I just mentioned, Demos work best when moving about, so the Camo netting becomes kinda worthless.
Pask: Pask is a good choice in any Russ, but he works best here in a Multimelta Demolisher, to help those High Str shots hit home. Tossing him in a Plasma Demo is even better, but the price tag would be obscene, and I cannot, in good conscience, recommend someone spend 250+ points on a single tank (which should prove that I'm not an SM player). The HB Demo needs him less, since you're putting out a good number of shots.
Now, how many of these terrors should you be taking? at 1000pts to 1500pts, I recommend a single Demo, backed up a regular Russ or two. Taking a pair is effective, though, especially when using mechanized lists. I would never council more then two in a normal game of 40k, though, as you'll want that last HS slot for something with a little more range and specialization, like a Vanq or Eradicator (two Demos and an Erad is a nasty combo, by the way). While the Demo is a good tank, it's also an expensive one, especially with the Sponsons, so going overboard with them is not advisable.
Demolishers are really about close support. Moving them up to support your advancing troops, or to blast the big nasty enemy units before they can get close to you, is what they're all about. Don't be afraid to push it forward into cover ahead of your line and let fly. They also work best if they only have one type of target to worry about. If you gave them Meltas or Plasma, have your supporting infantry pack HBs, so they can pop infantry while the Demo goes to work on tougher targets. The Demo is also one of the most effective tanks against Necrons. A Melta Demo can put lots of hurt on a C'Tan, and has a good chance of bringing down a Monolith (the main gun AND the lascannon both stand a good chance against AV 14). They also insta-pulp most of the Necron units, especially that pesky Lord. They're also great against Marines, where you can really lay out the pain on their high cost troops. Against Horde Armies, I would say take just one, as Exterminators, Eradicators, and normal Russes do the job just as well, for lest cost, and from further away (so they don't get punched to death).
So what are your thoughts on the Demolisher?