Well, you get what you pay for. But even top-price suppliers will make blemishes and leave them in, unless your spec says otherwise.
Typical example, PCBCart does what looks like a raster (inkjet) process, rather than "silkscreen" as such. It has reasonable resolution, apparent blocky edges, and a streaky scan (much as dirty nozzles in any inkjet looks). I haven't noticed any voids, but if you need a solid layer, for appearances, or for insulation, you'll want to request a second coat (nonstandard process, $).
You can also get multiple soldermask layers or colors, if you like; there's a lot of flexibility in PCB fab, but you have to be willing to pay for it, as soon as you stray the slightest bit from the commodity proto service.
Tim