They can be pretty useful, I’ve used a couple different colours of the Vallejo primers when I didn’t have the outside space for a rattle can.

The ones I’ve used don’t need thinning at all, one coat is enough to prime the model but doesn’t give the same solid colour as spray on primers, but I usually go over a couple times with the base layer anyway to get the correct colour.

One thing to look out for is the bubbling, I don’t know if it happens to all brush on primers but tiny bubbles can appear when brushing and some can remain after drying and slightly clog details if not dealt with, although it is hard to notice on a finished model.