I've read plenty of series where the subsequent volumes were as good or better than the first (and I don't mean mystery series where each book has its own story).
BTW I'm reading Peter Hamilton right now.
Yeah, his books are wordy and long-winded, and they will never be in my top 50 or even top 100 list, but I'm enjoying them, they're good entertainment. And that's mostly all I want from books these days.