I personally like to choose a broad category, say Home & Kitchen, and then write reviews on products in several sub-niches, such as Bedding, Kitchen & Dining, Furniture, Bath, Home Decor and so on, instead of just focusing on Furniture, for example.
My conversion rate hovers at around the 8-10% mark, so I don't think that focusing on just one sub-niche makes you look like anymore of an expert.
What do you feel the advantages and disadvantages of both methods are?
EDIT: I messed up. I meant a category such as "Air Tools" in the Home Improvement section, which features a range of different products, such as Air compressors, blowguns, caulking guns, nailers, saws, screwdrivers, sanders & grinders etc.