To make this segment a bit more bearable for me, I'm going to be just going with the flow. So apologies for the 'scattergun' approach in advance. I've decided to go for Centurions this time around. They're shiny and new and I've played against them a lot.
The main difference between their statline and that of a standard marine is that they have an additional strength and toughness and a 2+ save. That is it.
I feel as though Assault Centurions are easier to do, so I'll go with those. If you have a building or a vehicle, Assault Centurions will smash it. 3 attacks each on the charge with S9 Armourbane at AP2 will take it out. I have a couple of problems with them though. The most glaring of which is their inability to deal with infantry. 10 attacks in total on the charge from a 3 man squad. 5 hits, 4 wounds. If that unit has an invulnerable save prepare to see that go down to 2. Either way you're not killing infantry very quickly and if I have a vehicle that you want to get these guys into, I'll just bubble wrap it.
The second issue is transport, there are so many things that can just cut you down and every competitive army has a way of dealing with heavy infantry. Plus you're slow so I can just keep walking away from you. So really you need a Land Raider to speed you up and keep you alive. Now you're sinking over 400pts minimum into that squad. Not entirely sure how it's worth it? Especially considering how much damage you're putting out.
Essentially, I don't rate the Assault Centurions very much having played against them. The Devastators however are interesting.
Devastator Centurions come in 3 different varients. ranging from 200-290pts. The first variant is the standard. Centurions with a twin-linked Heavy Bolter, Hurricane bolter and Omniscope. The Omniscope is generally considered mandatory by most players, as it grants the ability to ignore night fight and gives you split fire. However, I'm not sure about it in this build, as really only the split fire comes into effect due to high AP. However 10pts for Split Fire isn't bad. This is probably my least favourite build though, as essentially you're not churning out too much damage again for your points. 9 twin-linked S5 shots isn't bad but considering how many points you're paying, why not 2 Thunderfire Cannons?
The second variant has a bit more hype. 3 Centurions with Grav Cannons and grav-amps, essentially the Omniscope is there again for the split fire and you keep the Hurricane Bolters. This is 60 more points than the previous variant. They're very good against heavy infantry and decent against vehicles with the Grav Amps, essentially, they are either very scary for you, or very much the opposite. For example, if you're playing against a Draigowing army and you have 18 of these, you're going to smash him, whereas if you're playing against the Screamer Star, you are going to cry because they don't have an armour save.
The other problem is range. If they are scary they are more useful in terms of board control rather than anything else as you can slap them in the middle of your deployment zone and dare your opponent to come close. Stomp up the field 6" per turn and slowly push your opponent back as they don't want to get near you. Of course, if your opponent doesn't care about them, this won't work. The Grav Cannons are highly situational.
My favourite loadout is the Lascannons and Missile Launchers. It comes in at 30pts more expensive than the Grav Cannon configuration, but I prefer it. It's no where near as situational, as almost every army nowadays either has a vehicle or a monstrous creature in it. In addition, it has a 54" effective range with the Relentless so it's much harder to hide and the ingore night fight rule on the Omniscope comes into effect with these guys. Yes, they are rather spendy for putting out damage, but they provide some excellent anti-vehicular support, and if your opponent does get close to your Thunderfire Cannons, they make an excellent tarpit.
In short, I feel that if you are going to take Centurions, and although you are not short of ways to customize them effectively, I prefer the Las/Mis loadout, although your mileage may vary depending upon the rest of your list. A well balance unit, but certainly not as broken as some of the rumour mills were hyping them up to be.