They are good. They can get about 20oc colder than ambient...so, when it's 40oC in your van during the daytime whilst you're out doing something, it struggles. I beleive it get about 40oC hotter than ambient - never tried it though. On my last trip for two weeks in the summer, it kept at around 7-11oC.
Check at regular intervals that someone has not accidently flicked it onto heat....as it makes a great seat, this can happen.
I tend to keep the van as cool as possible (park in the shade, use the blinds/curtains, park with the rear facing the sun etc).
Keep it full, but let the air circulate around (keep the vent clear). Supplememt from ice blocks that you can then freeze again on campsites / pubs / tescos etc.
Keep some paper towels in the bottom...to soak up condensation. Keep the lid open when you're not using it.
Drink ale instead of lager....you don't need it as cold.