I wouldn't worry too much about being niche specific at first. Many 
review style sites are not geared towards just 1 or 2 niches. If you 
can write compelling reviews and gain the traffic needed, the niche 
will not matter. Unless you personally prefer to target a specific niche,
I would concern myself with ANY higher end products more so than
a particular niche.
Successful review sites are only as successful as they are honest 
and must be full of useful, valuable information.
People can tell when they're being handed a bowl of bull-piss.
My wife is always telling the kids "Don't piss on my back and tell me 
it's raining", you know. If you are reviewing a product, it is best to 
be able to give first hand experience when doing so. This, usually, 
requires that you have actually USED or tested the product.
If you are providing a "blind" review, people WILL know, either by the
tone of your message or worse, the truth once they've purchased it 
for themselves only to find out it's NOT quite what you said it was.
The hardest part of review sites is that it's all about TRUST. When 
first starting out, you may be lucky enough to have some people 
take you at your word.
However, as times goes along, they will either start trusting you, or 
NOT. If you really want to make some good money at reviewing items
 to increase affiliate commissions, give 100% honest reviews, even if 
YOU feel that the product is not worthy.
I have given plenty of reviews which told my viewers exactly why I
did NOT like a particular product and many times, I'd receive emails 
telling me that MY dislikes is exactly what they LIKED about it, so, 
a negative review will NOT necessarily lessen the amount of earned
commissions you could earn.