Sentinel: We have two variants to use, the Scout Sentinel, and the Armoured Sentinel. Point to point, the Scout Sentinel by far is the most valuable. However, that's a little unfair, since both should be serving a different role in your army.
Scout Sentinels. These come with multi-lasers, scout, and move through cover. Being armor value 10, and open top makes them highly vulnerable to even bolter fire. A single Dakka Predator/ autocannon, with heavy bolter sponsons, can pretty quickly shred an entire squadron of these sentinels. So the worst set up I can imagine is putting heavy flamers on these. Ive heard of people using heavy flamers on them, but the idea of putting a squadron within rapid fire bolter range seems kind of foolish.
The scout sentinel is not a up close vehicle. So I prefer them with longer range weapons. Even the multi-laser is not long enough. Get yourself a weapon upgrade, and make it cheap. We only have a BS of 3, so lets fire twice in a round so we get a few hits. I prefer the Autocannon, and if you feel feisty put a hunter killer on them. If you go first, feel free to set them up along board edges of your deployment, but I prefer these units being put into reserve in order to outflank.
Outflanking is where these units shine. Cheap at 40 points a piece, 50 if you put the Hunter killer on them (I do), and make sure you have an astropath in your army. Fielding them solo, or in squadrons, they are then able to come and lay down a torrent of fire into the side armor of vehicles. Their job is simply fire suppression and if you know your going to have lots of objectives all over the board, use them to eventually move towards near objectives for contesting.
Armoured Sentinels: Having their origins back in Armageddon back in 3rd edition, I loved the way these look. The armoured sentinel doe not fill the same role in my playbook. It is not a flanking vehicle, nor does it come with scout ability. It is armor value 12. This places it into your front lines, or at least just behind them. They fit wonderfully behind a chimera chassis, and this is where they should be. Once again, I prefer the Autocannon upgrade because of its low point cost, and a hunter killer missile. Reasons why? The IG is a round 1-3 army(not always but most of the time). Meaning we need to do well early game, or we could be in trouble.
Armoured Sentinels are nice, as most general troops do not come with a hand to hand weapon able to kill your walker. Powerfists even need a 4 to glance them, so would need to get lucky to take your walker out quickly. So a sentinel is a great way to hang up an assault squad (not terminators). This is not something to set out to do, its simply a last minute defense. Your sentinels should be shooting all game.
Armoured Sentinels should be providing a hail of fire with autocannons, as BS of 3 and lack of twin linked weaponry reduces its role in precision tank killing(don't put lascannons on em). Get a vendetta if you want that. Plasma Cannons are the other option. I don't use these myself, but the idea of 3 plasma cannons is a great anti-infantry gun for the armoured vehicle.
Keep your armoured sentinels behind chimeras, advancing with them and firing all game, and you will be happy with their results. Autocannons with Hunter killers, or Plasma Cannons are in my opinion the best option for them to get results. Moving along the front lines, in and out of cover amongst your chimeras is where you will find these in my army. I dont expect them to kill a ton, what I do expect is for them to lay down fire suppression, taking on the role of the weapon your put on them (autocannon or plasma cannon).
If you have used Sentinels, let us hear how it went. Most people think they suck, but they are expecting more out of them than what their role is, and comparing them to a Valkyrie or Hellhound. To make that comparison go point to point.