disc brakes start squealing after just a bit of use

A couple things to look at:

• Perhaps try Brake Clean to clean the rotor and spray on a rag then wipe the pad surface. I successfully use it on rim brakes when they occasionally squeal and it is not due to the toe angle of the pads. However be careful with it as it is stronger than rubbing alcohol so do not get it all over the place.
• It could be possible the rotor is out of true. In which case, you should go the shop you got the bike at to get that addressed.
• You could possibly have a bubble in the brake line which is causing a lack of pressure to close the caliper and that in turn is causing vibrations as you slow along with the loss of braking power. Again the shop that you got the bike at should be able to address that for you pretty quickly.

Hope that helps.

Have you ever listened to what a racket the rim brakes on comp trials bikes make?

This is by design: comp riders roughen up the rims so the brakes will lock up better. Deceleration, what every other discipline uses the brakes for, is hardly relevant for them.

Now, I actually have the Varial Hybrid myself (since November), and I too was surprised by how squealy the brakes were to start with, but I assumed it was also intentional (though I couldn't really think how they would have done this with the disc brakes).

By now the brakes sound just like the ones on my hardtail MTB (which makes sense – also Shimano 2-piston with resin pads), and they brake similarly too. Unfortunaly they also don't lock up as promptly anymore, but if you're more into street-style riding that's probably fine for you, so just be a bit patient.

Of course it's possible that your brakes really have an unrelated issue that my Jitsie doesn't have, but before you worry too much give it some time first. You're (hopefully) not going to send big downhill tracks on that bike, so if the deceleration isn't optimal you should still be ok.

It sounds as if you are glazing your brake pads.

Pads usually make noise when they are brand new, contaminated or glazed. If you do not break in your pads adequately, there is a great chance they will become glazed. This can be caused by braking hard with new-ish pads. The heat will polish the surface and greatly reduce the braking power, thus requiring you to apply more power on the levers and heat building up.

Non mineral oil has a tendency to boil when heated up, i.e if you brake hard or if your brakes heat up too quickly. This results in the lever becoming mushy and the braking power reducing significantly until things cool down again.