I actually enjoy reading reviews that appeal to my interests, but I'm far more likely to really read it and follow their advice if it comes unsolicited or unpaid-for.
I also like when blogs do give-aways, but if every other post on a person's blog is them giving away stuff, or trying to win a giveaway, and that begins to overshadow their blog's actual content, then I lose interest pretty quick.
A few here and there wouldn't hurt. If you try to keep it blog relevant it's even better. When Blogs i read do this i feel pretty happy for them - they are doing something they love and getting reimbursement and recognition. That's never a bad thing.
I've been wondering about this myself and I definitely think it has to be something that you would be using or interested in using anyway.
I also think that if it happens to regularly then one's blog loses it's personal aspect!
I think if you space out the sponsored posts, do one maybe once every two weeks, or every week at most, but still continue to have decent, creative posts by YOU, then it's not a big deal. But I will admit, that if all a blog has is flashing, distracting ads, and posts that only push products at you... I will just unfollow. I don't think anyone can blame you for wanting to earn money, and I plan on trying to monetize at a future date. I just think you need to find that balance that people can live with. But there might be a few people who unfollow... but as long as you keep your standards, you'll find more readers and keep the readers that truly matter.