So far testing Mega Steelix is going very well. I'm still makin my way up the ladder because ever since the ladder reset upon ORAS' release I only played on the OU ladder using a total joke team with 5 trappers and 1 MEvo hahaha. Before last night I had played in multiple mini tournaments hosted in the OU chat room but that's it, though I digress. The most effective way I've used Mega Steelix is basically hit shit hella hard with sand up and whenever sand is not up you can use it as a fantastic bulky pivot to take a hit easily, scare the enemy out, and predict what they will switch into and pull a double into the appropriate 'mon, unless Mega Steelix is capable of smashing the enemy's switch-in without sand necessary which I've found to not even be a very uncommon occurence since Mega Steelix's most reliable STABs, Heavy Slam and EQ, have very high power and p good coverage too. The only time Mega Steelix fails at doing anything offensively, keep in mind that its offensive prowess as compared to Mega Aggron is what makes it viable at least in my opinion, is when the opponent has one of the select Mega Steelix hard counters such as Rotom-W and Skarmory. Switching into said Pokémon, however, is extremely obvious on the opponent's part so such situations can be exploited by you. Although guessing games created by the need to switch into one "obvious" Pokémon are technically in neither player's favor and thus it can work for you or against you, you must keep one thing in mind which is that he is at a greater risk than you because, prior to the turn in which he may or may not switch into Skarmory or an other such Pokémon on you, you are the one with the initiative and you risk merely losing initiative if he wins the mind game whereas he risks potentially losing a Pokémon altogther if you both keep your Pokémon in battle and Mega Steelix beats his 'mon to a pulp. Anyways as I was saying earlier, when sand isn't up just use Mega Steelix as a worse Mega Aggron(keep in mind that it can perform Mega Aggron's role as a tank extremely well, just not as well) and when sand is active then use it as a tank and hard ass fucking hitter.
People are always comparing Mega Steelix to Mega Aggron, but that's not the only Pokémon to which it should be compared because of Mega Steelix's characteristics and what team archetype it is to be used in order to be used to its fullest extent. Consider it as a Mega Aggron-Excadrill hybrid. Mega Aggron defensively outclasses Mega Steelix in every way except for Mega Steelix being capable to take neutral and resisted special hits better and being a check to Volt Switch spam. In return, Mega Steelix completely outclasses Mega Aggron in offensive prowess. Excadrill, on the other hand, outclasses Mega Steelix in every way offensively except that Mega Steelix has potentially substantially higher powered Steel-type STABs. Duly note that Mega Steelix's Steel-type STABs are an offensive advantage as Heavy Slam combined with Mega Steelix's bulk allows it to let even Defensive Landorus-T not be safe switching-in because it'll be 3HKO'd by Heavy Slam and EQ will fail to 2HKO Mega Steelix. Heavy Slam also OHKOs the Lati@s' base formes which Excadrill's LO Iron Head fails to do. Think of how hard LO Excadrill's Iron Head hits, now imagine giving it a second Life Orb and taking no Life Orb recoil from either of them. That is Mega Steelix's offensive prowess in the sand.
I have also made good progress in determining what is and isn't viable on Mega Steelix. Max HP max Atk RestTalk or max HP max SpD RestTalk are honestly the only sets worth using. There's virtually no reason in using a 3 attacks+Stealth Rocks set because then you might as well use Mega Aggron because Mega Steelix will just die mid-game due to the lack of recovery and then you will either hardly or not at all be able to take advantage of what is the entire reason to use Mega Steelix over Mega Aggron which is its capability to hit hard as fuck and Mega Aggron would set SR more reliably anyways, not only that but if you're using Mega Steelix you should be using a Defensive Tyranitar or Hippowdon with Smooth Rock and you could just have Stealth Rocks on them instead of on your Mega Evolution. Another thing I have to add is Heavy Slam is 75% of the time > Gyro ball despite the higher potential damage output because running minimum speed for Gyro Ball will not allow you to outspeed and 2HKO a weakened Mega Sableye because with minimum speed you will get outsped by Mega Sableye of all things and end up gettin' burned before you can finish it off and then it can Recover in your face. I even ran into one scenario where I managed to severaly weaken a Mega Slowbro trying to set up on my and had I not been running minimum speed then it would have been a speed tie and I might've potentially knocked it out with EQ, so now I actually run 4 speed evs to speed creep Mega Slowbro if there's ever that rare circumstance appearing again. I might as well try and speed creep Mega Slowbro anyways too because putting 4 evs anywhere else is completely arbitrary whereas those seemingly arbitrary 4 Speed evs actually help in at least one situation haha. Lastly, as I displayed in calcs earlier in this thread, Gyro Ball fails to achieve sufficient Base Power against Defensive Landorus-T to 3HKO and thus Defensive Landorus-T is a counter to Mega Steelix w/ Gyro Ball. The only real benefit of Gyro Ball over Heavy Slam is having a chance to OHKO Scarfed Landorus-T after SR damage, but that's only a chance to OHKO so I'd much rather use Heavy Slam to beat both Landorus-T sets. Higher Base Power against fast, offensive teams is not really worth it when Heavy Slam in the sand will OHKO(oftentimes sand isn't needed either) any offensive 'mon that doesn't resist it anyways.
Sorry for the huge essay haha, TL;DR: Mega Steelix is proving its worth easily and needs an analysis, but if you still don't believe me then replays are coming soon.