I like to be honest on this channel, so I'll give you my honest peer on online mastering. I actually have a little bit of history with online mastering. In the early 2000s, I developed the first online mastering service, which was called E-Masters. I kind of started that whole online sending your tracks to be mastered, before all the other studios jumped in.
Really, online mastering has come a long way today. But the reason why it's so good now is that so many people are working in their home studios. When I started, they weren't. Today, however, you can send your track off to one of the top mastering guys in the world, who has a room and speakers designed for that purpose, for as little as £50 to £200.
Not only that, you have the liberty to choose who gets to work on your track today. You can browse through a list of candidates, and look at the stuff they've worked on, genres that they're good at, and make your choice from there. That is great because you've got a new set of ears that can come back and say, "Have you thought about tweaking this?"
Or they might say, "It's great," which gives you the confidence to think, "Okay, the room's going well, and I'm doing alright with the mixing." It's just having that extra person there who's a professional who understands what things should sound like when they're finished to help you progress.
I think that's very important rather than trying to record, mix, and master yourself. From my experience, I will always get a better sound, get it more open or louder, if that's what they want, 75% of the time. That's because I always have that different perspective on it. I'm not listening to the individual instruments; I'm not worried about what the problems were with the mixes. I'm just taking it at face value and then reporting back to the mix engineer, asking for tweaks, and then getting it to the place where I want it to go.
So, yes, it's worth doing online mastering. It's worth doing it because it's so cheap compared to if you have a mix on that'll cost you £500 to £1000. It's literally 10% of that. So give it a go; try people out.