I owned a DG-300 for a good while, and really liked it - many memorable flights including the 1000-km task during the Regional contest at New Castle VA. I'd recommend it highly.

Here are two notable details worthy of attention, that seem not to be common knowledge among owners:

The gear-retract handle is a knob on the end of the actuator rod that rotates outboard to lock the gear up or down. Its rotation is restrained by a friction washer. Over time, this wears / compresses, so the friction decreases. It can reach the point where the knob rotates freely, making the gear somewhat prone to collapse, especially when landing on rough ground. The friction is easily adjusted by a nut that compresses the friction washer. Owners should be aware that a freely rotating knob is a problem waiting to happen - but it seems only a minority understand this issue.

The spoiler panels are often just a bit too high at their outboard end. This means that when you pull up and the wings flex, the spoiler box pushes the end of the spoiler up a few mm above the wing surface. You thus suffer a small performance loss each time you pull into a thermal, and perhaps when you thermal in a steep bank.

The problem is easy to detect: Pull the divebrakes open and inspect the bottom of the box at its outboard end. If it's been pushing on the spoiler panel, you'll see a black mark caused by aluminum rubbing on fiberglass. In my experience, about 60% of DG-300 gliders exhibit this (I have no info on the DG-303). The solution is fairly simple: shave a small wedge of aluminum off the outboard bottom edge of the panel - perhaps 6mm at the end, tapering to nothing over a distance of 80mm.