What to look out for.
1. Manufacturer. Most batteries (Including Genuine batteries) are made in China. You will find that virtually all of the compatible batteries are made in China. This is not necessarily a bad thing, many factories in China have achieved top certification and QC even by international standards and can manufacture quality products (ie. Apple iPhones, iPods... etc). What to look out for is the branding. Many battery manufacturers do not put any branding of their batteries, this is very worrying as the manufacturers do not want to take any responsibilities in the event of any damage. Look for a battery that has the manufacturer and branding well in view of the user. Maximal is a reputable aftermarket brand and provides the longest warranty (3 Years) and also a $1,000,000 USD Equipment damage cover caused by the battery.
2. Capacity. The label which is printed on the battery can be anything made up by the manufacturer. Because their name is not printed on the battery, they are happy to put anything that will give them a sale. Always trust the brand that states the mAh which is identical to the original battery. This will ensure you do not damage your Camera.
3. Equipment Damage Cover. A handful of battery manufacturers provide this and is fairly useful for some peace of mind.