In my experience with Goodreads, it can get you reviews, but the percentages can and will vary. I did three. One was a US giveaway of ten copies, and maybe half or so reviewed it. The second time I did another ten copies and expanded it to the world, which was more expensive due to the postage fees to ship overseas, and as I recall not nearly so successful in generating reviews (can't say how much or how little that might have been affected by the fact that a good number of those winners lived outside the US). Goodreads claims an average review rate of around 60%, but even if true not every giveaway will earn that kind of return. Last time I did one copy, just for exposure's sake. Other than getting on some more folk's To Read'lists, I didn't notice anything special from that one.
I've been tempted to do what someone has been doing with one book, but I can't decide how I feel about the tactic. The book is a perennial on the giveaway list, always appearing at the tail end of a given day's giveaways, having been entered a day or two prior to that day's cutoff. Nonfiction. I do remember reading a mention of it in the book section of Entertainment Weekly as one of a selection of up and coming books, but no idea whether the frequent giveaways may have had anything to do with that or not.
And while a giveaway may get you on a lot of To Read lists, I've noticed that some of those folks have thousands of books on their lists, so whether or not they'll ever get around to ours seems problematic at best.