Extremely accurate, if you've calibrated them correctly!
Mine is off by less than 0.4% - I've measured. That's less than 2000 ft off after a century (100 miles).

Brand of computer shouldn't matter, at least not if they're simple enough in their design. The more complicated they get, the more things that can go wrong...

One thing to think about, though, is how precisely you are able to set the wheel size. Some models only allow you to set a certain, pre-set, wheel size, such as maybe "25-622" or "47-559". These models would be slightly less accurate, unless you happen to pick a perfect "fit" for your particular wheels. But they'd still be accurate to within maybe 1-2%, and that's not too bad.