Honestly, it's so strange this never comes up - yes there are ads but a man's gotta eat. The ads aren't particularly intrusive so the free version is a fine sacrifice for those of us who are happy enough with the base functionality of sync and can deal with the minor annoyance of an occasional ad.
I'd prefer to purchase the ad-free version, but the pricing is a bit excessive for me right now - I can wait it out until there's a sale or other discount in the meantime.
If that's a dealbreaker, all the other Lemmy clients are available to use instead - I've used them all and they're all excellent.