I have no personal experience of using suction-cup window feeders. Although, using any feeder, it always takes quite along while for birds to discover the feeder.
There are many customer reviews of suction-cup feeders at Amazon. There are many models. The majority of customers seemed to be very pleased with the results. Examples:
Not all birds want to use vertically mounted feeders. Tits and finches of all kinds do, though blackbirds and robins prefer to feed from a flat surface. The Meripac (2nd review above) looks as though ground feeding birds might use it.
There are several other designs on Amazon. You can see them in the 'Customers who viewed this item also viewed' sections on those pages.