Pros for walkers
First off of course, we have the change to the damage tables, meaning explosions are rarer. So, walkers will last longer.
Secondly, they now pick up hammer of wrath, so when charging you will likely be doing extra damage, particularly given the higher strength of most walkers.
Thirdly, their rear armour is much safer than most vehicles. Yes, you can still be shot in the back, but unless you're immobilised, assaulting units will only hit your front armour (just watch out for melta bomb equipped sergeants!)
Fourthly, charging units aren't safe from overwatch - walkers can fire overwatch, and more importantly, can do so even if the charging unit is outside it's usual fire arc (unless immobilised). Time to dust down that twin flamer load out! Many units will think twice about charging a unit capable of putting out twin walls of death!
It's a fairly short list really, but they are still vulnerable to high strength shooting from the rear, as few have heavy armour there. Secondly they are pretty hampered by most of the penetrating hit results on the damage table, since a lot of walkers tend to be heavily armed, and limiting them to snapshots really hurts their effectiveness.
So, what walkers would you take and how to equip them?
You've basically got three types here, and the reasoning behind the bog standard dreadnought hasn't change much, I'd say there are two really viable builds. The first is the suicide melta dread, equipped with a multi melta and in a drop pod. It arrives, let's off the melta shot and if it survives the return round of shooting, can really put a spanner in the strategy of your opponent. Second is the rifleman, with two twin linked assault cannons. It's basically a quad gun that can shoot at ground targets, but it's decent for stripping hull points from lighter vehicles.
Second on the marine list is the venerable dread, now a lot of people really didn't like this option before, but the extra points give you a noticeably better BS, and now that exploding only happens on a 7+, being able to force your opponent to re-roll those results is massive!
Finally we have my favourite, the ironclad. An extra point of armour on the front and sides means this baby isn't going to be hurt by much. I like to drop mine in amongst the enemy lines with twin heavy flamers and see what's still alive on turn 2 to charge.
Ok you think the marine dreadnought is good? Try the Triarch stalker - it may only have one weapon but the basic armament is either a two shot multi melta or a heavy flamer (and can choose to be either during the game) or you can give it a large blast one shot autocannon, or (effectively) a lascannon. Added to that, any hits it scores on a unit means other Necrons shooting that same unit are twin linked. Oh, and it ignores crew shaken/crew stunned results on a 2+/4+.
Did I mention it's quantum shielding gives it 13 front and side armour until a penetrating hit?
Well for starters they come with an invulnerable save and can put out an obscene amount of firepower. The weapon systems are not twin linked, so firepower is potentially doubled. For ten points it can also get the 2+/4+ to ignore shaken and stunned results, and these guys are fleet scouts with battle focus, so can run and shoot in the same shooting phase after amending their deployment to better counteract the enemy. Weapon variations are cheap too, and the scatter laser option gives you twin linked on the other weapon provided it hits at least once.
So there you are, pros and cons of walkers and a brief rundown of some of the examples. I'm predicting we'll see them much more in this edition, I know I'm planning on expanding my collection!