Aftermarket batteries for dSLRs - much less expensive, but are they good?  The right ones work as well as Canon’s - but how do you pick the right ones?  The bad ones are a bummer.
   Here’s my experience; go to Adorama’s online store - and read what prior buyers say.  You’ll fast learn - as I did before buying batteries for my Canon 7D - which aftermarket batteries are as good as Canon’s, and which only cost less.  And the differences are stark in terms of what prior buyers say.
   Adorama’s online store has detailed ratings from prior buyers for aftermarket batteries - and for all sorts of products, whether those of a big-name camera maker or aftermarket.
   Don’t bet on one brand of aftermarket batteries to work equally well for all cameras; so check aftermarket battery ratings for your particular camera before buying.  
   One thing is for sure; you really can save a lot of money by buying aftermarket batteries - if they’re the best ones.