Ok I have had a play with quite a few of the new ORAS megas and wanted to give my thoughts about the ones I have used.
Sableye is a horrible troll on your opponent's team and can be a win condition if they are too slow to deal with it. Prankster burn and recover makes its base form hard to break by its own right. I found the best strategy is to burn and then switch between healing and calm minding. Once you are happy with the boosts and are healthy go Mega and start sweeping. Its best to not go Mega straight away (unless you need magic bounce desperately!) otherwise that slow speed could be the death of Sableye. Oh and it also hard counters baton pass teams which makes me very happy :-)
http://play.pokemonshowdown.com/battle-orasou-177643789
Salamence is just overwhelming. It can 6-0 teams with ease and it is so hard to stop it set up. Its the best OU pokemon without question and unless you have at least 2 checks (no counters cause its so versatile. And most of its checks are dependent on the moves Mence is using) and use them perfectly you are going to lose.
Slowbro can be just as dangerous as Mence if it can set up, fortunately its special defense is vulnerable until it has 3 calm mind boosts so powerful attacks such as Mega Sceptile's leaf storm can effectively OHKO even at +2. I am experimenting with a calm mind, amnesia, scald, rest set right now. The tactic is using amnesia to quickly boost that sdef until you can't be touched and then slowly calm mind until you sweep. No rest talk is a issue though so set up sweepers can give it problems. But as a end game winner it really excels.
Beedrill is ok. Its very dangerous when it evolves but it has serious 4MSS. I know protect is the common move these days but really I prefer not using it and U-turning on a slow mon instead. I don't think he will be a high OU mon, but its power once given the right support (i.e. no rocks on the field) is serious.
Metagross is not as good in practice as it is on paper. It hits hard but just not hard enough to get a clean KO. It has great bulk but lack of recovery means it still gets quickly worn down. Its a solid OU mon but I can't see it being top tier OU either. I have never done a full sweep with it nor has anyone ever done that to me. Swampert is in a similar boat although it does fair bit better than Metagross. It can 6-0 your team, although it needs to have someone else set the rain for you and to be in Mega form once it comes in. Until those conditions are met it can be crippled or lose enough hp so that it can't do its sweeping job. Its best left to the end of the game where your opposing team is not at full hp and your have already Mega evolved (but preserved its health). Under those conditions its a game winner.
Gallade is a monster and IMO the next best Mega after Mence. After one SD it has enough power and speed to crush most things and unlike Metagross its can hit most things for neutral damage. Lopunny is also a great physical attacker that can 6-0 if given the chance to abuse power-up-punch. Most of the time however its a hit and runner and has enough power to punch big holes in their team until it can sweep. Gallade and Lop are the dark horses of the new Megas, once the dust settles and the OU metagame is balanced these 2 Megas will probably be sitting somewhere in the A-S viability range. They are really good.
Sceptile is a bit like Mega Skywalker. You have no idea if its physical or special (both are viable) and it has plenty of speed and power. While it hits no where as near hard as Luke and has no priority its much faster and can't be crippled by thunder wave. It will be a solid OU choice.
I need to give the other Megas a go before passing judgement but all the ones listed here should be OU (and some might be higher...).
