We have one, it’s OK….stinks out the kitchen (windows open and avoid going in the kitchen while cleaning) It tends to leave a residue on the glass, which is hard to remove. Our’s doesn’t get the oven ‘clean’, but it does burn off the stuff that smells when you cook.
Think it might have needed the mains uprating to install it(?).
I find it useful, but wouldn’t be fussed if we didn’t have it. A lot of it depends what you cook, if you only heat up pizza’s or do cassoroles, it’s probably not worth it, but if you do a lot of roast dinners / Yorkshire pud’s it’s good for burning off the fat which tends to splatter onto the oven walls.