As I see it, getting paid for reviews can be broken down into different scenarios:
- You work for someone that pays you to review other products i.e. you're being paid by a neutral party with no affiliation to the product itself.
- You are approached by someone with a vested interest in the product, and are either paid to review that product, or get given the product for free, but the expectation is that your review is your own, and in no way needs to be positive or negative.
- Same as above, but you are expected to write a positive, glowing review.
In the first scenario, this is generally okay. Many companies - consumer magazines, newspapers etc. - do exactly this. Whenever there may be a conflict of interest, it's always wise to be transparent and reveal it.
In the third scenario, this is clearly unethical. It is tantamount to a bribe - "we'll give you money if you say something nice" - and it's for this reason that such reviews are viewed as being unethical.
The second scenario, especially if you're getting paid, can be a grey area. For example, you may be more inclined to say something nice, or to tone down your negativity, in order to ensure you get similar work in future. If you are really critical, you essentially shoot yourself in the foot because no-one's going to come back to you if you're posting negative reviews about their products, and no-one else will want to approach you to review their products at the risk of you saying something negative (unless, of course, you have such a large readership that it's worth the gamble).
As an aside, I used to DJ and get free promo records from companies, and in return I needed to give feedback. After being brutally honest with a few reviews, I ceased receiving any more records from certain labels, so the pressure to be more forgiving and positive is real.
The question you have to ask yourself is, what is the purpose of the reviews for you and your business model? Is the revenue to come from the businesses in question, or is it to come from increasing your profile and build your readership to make more money from them (e.g. donations, clicks on advertising on your site, purchase referrals etc.)?
If it's your readership that you're focusing on, then it seems obvious that you should maintain your impartiality, and not accept payment for the things you review. You need your readership to trust you. In most cases, getting access to the product for free (e.g. getting a free book, access to the service etc.) should be enough. If you build up your readership, and become an influencer because you're honest, constructive and transparent, you'll find you're more likely to get access to more free stuff. I would also recommend always mentioning that you were given the product for free in order to do the review.
If the revenue is to come from the businesses, and you can't think a way of changing that business model, then at the very least you should be transparent by revealing to your readership that you were paid to do the review, but I wouldn't recommend this approach, since the question always lingers as to whether or not you were truly impartial.
As a reviewer, all you have is your integrity and honesty. Lose that, and you lose your readership. They'll no longer trust you, and that can lead to the end of your online profile.