So I know we already kinda had the trapper-viability discussion, but I'm going to go with this either way, since I see the overall discussion has slowed down a bit. I'm going to bring up the unique niche of Magnezone trapping Steel-types. Although not the only role Magnezone can take, it is most propably the most common one, and the biggest reason why one would use Magnezone in OU. The reasoning behind my arguments is going to stem from the official OU analysis in the making. I don't really have strong opinions about Magnezone's rank, I just want to see it ranked. For the sake of discussion, I am going to nominate Magnezone for the rank of C+ along with some other trappers, for example Dugtrio.
The biggest reason to put the + is that Magnezone's niche is unique. There are no other Steel trappers in OU, and Magnezone is even quite good at what it does. Magnezone benefits most from teams that like to have Steel-types removed, and it works kinda well in this role still. The traditional type Magnezone was paired up with was Dragons, and they still benefit from removing opposing Steel-types. However, now there are also Fairy-types to worry about. Despite not being able to trap Fairy-types, Zone still has a pretty good matchup against most Fairies, so Magnezone can still work quite well on teams that depend on their Dragons to make havoc. HP Fire is quite the sure kill on most Steel-types of OU, and Magnezone's defenses guarantee that unless opponent is carrying a coverage moves, they won't succeed in killing Zone. Also, there's now no permarain that can take away from HP Fire's usefulness, although the overall HP-nerf is a bad thing for Zone. One problem to using Magnezone is that there are still Steel-types that Magnezone cannot revenge kill, and the trapping should only be done on mostly defensive Steels, eg. Skarmory and Ferrothorn. It can also defeat some attackers, like Genesect and Scizor, but that depends on their coverage moves. Most problematic Steels are Aegislash, Heatran, Exca and (+2, not so sure about unboosted) Lucario, who can either kill Zone reliably or escape his Pull all together. Also, being locked into a weak HP Fire is not the best way to start a round after the opposing Steel-type has been demolished.
Steel trapping is a unique niche granted to Magnezone, and it can work in it. However, the unreliability against the most common Steel-types is really offputting, and makes you wonder if Zone really deserves a rank just because it can reliably defeat only some Steel-types. Now I would like to refer to the Specs/Analytic-set, which, for me, has worked really well. Magnezone has a plethora of resistances, and it is not hard to find room for Zone on an offensive team. SpecsLytic (I just made this up, sorry) hits incredibly hard, and by using Volt Switch, u become arguably the hardest-hitting pivot in the game. Unless your opponent has Ground/Fighting/Fire-coverage, he is almost forced to switch, or atleast take a truckload of damage from this behemoth. The one downside to using this is that Volt Switch is the least spammable alternative of Volt-turn core. There are Ground-types behind every corner, and they'll just love to switch to Magnezone to take him out. Now the outcome will depend on your coverage moves. Flash Cannon hits Ground switch-ins from some serious damage, although it will not OHKO everything. Hippen Power Ice becomes alluring, since that's a great way to get rid of those pesky Grounds. There are still some Pokemon who can switch to Zone, and maybe even take 2 hits from him. Exca doesn't fear anything, since even with HP Fire, u won't OHKO, and Exca can proceed to Earthquake. Aegislash and Heatran are both so specially bulky, that u just won't 2HKO them. Keeping up hazards can alleviate this problem.
K, so putting it all together: Magnezone has a unique niche of successfully removing some problematic Steel and Fairy-types from the game, and letting, for example, your Dragons come off a bit easier. However, Zone cannot reliably kill some of the Steel-types that are at the very peak of usage. If not going for Magnezone's steel-trapping capabilities, you might also want to consider the Specs + Analytic set, that has the merit of being the strongest Volt Switch in all of OU, letting your team grab momentums that much easier. Because of Magnezone's defensive merits (stemming mostly from the awesome typing+ nice defensive stats) and offensive capabilities explained above, I would like to see Magnezone ranked at C+.