Comes down to proper preparation and application. Follow the instructions patiently and carefully!
I used Ronseal and a decent brush (I vaguely recall it recommended a roller but I tested both and wasn't happy with the finish from the roller or how to blend the edges neatly) .
I'm really happy with it. If you inspect it closely you can tell it's painted but I couldn't care less because it's infinitely better than what I had before and I love the colour.
I didn't use a grouting pen afterwards as I thought they looked a bit shit (and they can run off when you clean the tiles). I like how it looks without and it's been hardwearing.
Worth buying a sample tile or two so you can have a little practise and make sure you want to take the plunge, but bear in mind painting a loose tile laid flat on a table won't be the same as painting multiple tiles affixed to a wall!
Think carefully about the edges. If there's an edge of your tiles with sealant the paint won't stick to that (unless it happens to be a printable sealant I suppose, but they're usually only for internal door frames etc not areas that get wet) so you'll probably need to remove it, paint them, let it cure, then put sealant down after.
If you use a brush make sure you're holding and applying the brush properly (i.e. Not by the handle), not stroking it savagely back and forth. And be careful not to overload it with paint or it will sag on the tiles. (Even anti-sag paints can't cope if you really trowel it on!)