How much does Assist Research actually help?

The ships give a boost of 5% per level of the commanding scientist to the planet that's being assisted, which increases to 10% with the Improved Assist Research technology. The assist research mission also steadily gives the commanding scientist experience, and so is a good way to prepare younger scientists to take over once there aren't any more systems to survey.

I've typically only found this useful with dedicated science worlds, like the homeworld (if you decided to build ~4 labs on it) or any world with primitive civilizations where you built an observatory enclave (because those can often produce massive amounts of social research). Most of the research income of a large empire will come from research stations, which this can't improve.

It really depends on how much research the planet is producing. For a specialised research planet with 20+ Science Labs, 20+ synths, and 150+ science output , it can really make a huge difference.