On the surface (or below it for that matter) the new Necron Canoptek Tomb Sentinel seems exactly like its cousin the Tomb Stalker. And while to two do remain many similarities there are a few big differences. The tomb sentinel itself is a very tough machine. Toughness 7, 4 Wounds, 3+ save and a Monstrous Creature is very impressive. It also has the War Construct rule which means that poison and fleshbane are only wounding you on 6's. Cry me a river Dark Eldar.
Now here is where it begins to deviate from a a Tomb stalker. First it is a whopping 10 points cheaper. The next is that it occupies a fast attack slot unlike the sentinel which takes an elite ( really FW?) The fast attack is already a very tight space with wraiths, scarabs and the new Acanthrites. So this is already in some bad light. It also only has just 2 attacks but it is initiative 4!
The biggest difference and the main reason you would take this is because of the exile cannon. The exile cannon is a 12inch( again FW, really?) Small blast. Models that are under the template must take a Strength test or be removed with no saves allowed. Vehicles are automatically penetrated.
While this seems good it really is not. Unless you know you are going to face Elder or Dark Eldar, then these things just won't be that effective against anything else. The gun is too short of range to really be a threat and since it has to get that close you are going to get shot with plasma or melts long before you are in an ideal position. Though it does have outflank, and Deep strike (lol) so if you don't give a damn and just want to throw caution to the wind go for it.
In conclusion this is not a very efficient unit. The Necron codex thrives on its unparalleled efficiency from most of its units but this one just doesn't fit. I wouldn't take one in a competitive game but maybe in a casual game.
Also the model is pretty nice. But it can also easily double as a tomb sentinel if you want. I doubt most opponents would even notice a difference between the two anyways.