Most shops will service off brand octos, but they may not be able to provide free parts under warranty. In most cases the average OW octo is pretty generic in terms of the seat used, so it's not a major service issue, but using third party generic seats can be a liability issue for a shop.
As noted above, life is easier in terms of servicing if all the stages on the reg are the same brand.
In terms of quality, I like to ensure the performance of my back up regulator is as good as my primary regulator. Something to consider is that in a real out of air situation, you are quite likely to end up getting mugged for your primary and ending up on your own octo.
Even if you are superbly trained and disciplined for an OW diver, it's just a reality that divers get separated enough that by the time you notice you are out of air, your buddy has swam on and you're now playing catch up. When you finally do catch up, there won't be much of the signaling out of air and waiting for permission business. There also won't be much time looking for the octo somewhere on your buddy, and the temptation will be incredibly strong to go for the reg in your buddy's mouth. It's a strong argument for the long hose primary and primary donation.
If you're in doubt of that, have your buddy swim at a normal rate way from you. When he gets about 30' away (assuming you have the viz) partially exhale and stop breathing and try to catch him to share gas. Leave the reg in your mouth as the odds are good you may need it to take a breath.
In terms of octo quality, having an octo that is very responsive and can deliver a lot of gas to a severely short on air and near panicked diver is a good thing to have. And, in the event the diver goes for your primary, murphy's law dictates he'll do it just as you finishing exhaling, so by the time you locate your octo and purge it, you'll appreciate a decent performing buddy as much as an OOA buddy would.
Leave the cheap bubble packed octos on the sales rack and buy one as good as your primary.
