When buying a table tennis blade, the rule of thumb for the choice of the right handle is usually the following:
– Straight handle: For those strongly grabbing their bat.
– Anatomic handle. For those loosely grabbing their bat.
– Flared handle: For those grabbing their bat not too hard not too loosely.
In addition, if you are a defender, the general recommendation is going for a straight handle as, if you use 2 different kinds of rubbers on each side (backside and pips for instance), this type of handle makes easier switching the sides of you racket during the rally.
Most of the time, and especially if you are a beginner, you will end up with a flared handle that is an average choice that improves manoeuvrability in the execution of the different strokes. It´s not a bad choice, in fact, in my case this was also my initial choice.
Moreover, after some time, I changed my mind as I found the straight handle more and more attractive. In fact, to me, all table tennis blades start looking better when they were presented in their straight handle version in the catalogues. At the beginning I consider this as a matter of personal choice but after a few years exclusively playing with straight handles I found a reason for this change: straight handle maximizes power looping. In my case this statement can be considered a fact either when I play with my forehand or backhand but, until now, I could not find a logical explanation. After a thorough analysis I finally identified some arguments that might help understanding why the straight handle helps the power’s transfer to the ball:
– “Your racket as the continuation of your arm”: The straight handle definitely helps on the accomplishment of this axiom by facilitating bending your wrist down to make your arm, hand and racket get aligned in a straight line. This principle is key to improve the power transfer at the moment of the ball’s impact.
(Image from http://www.allabouttabletennis.com/table-tennis-grip.html)
– “9 o´clock to 12 o´clock rule to brush the ball”: This is a principle that is difficult to explain in the short extent of a blog entry. Nevertheless the following video from Sean O´Neill – a former USA table tennis champion and great trainer – can help to introduce this principle (at 3:25):
http://www.youtube.com/watch?v=e0AJmusJasA&feature=player_detailpage#t=205
The position at 9 o´clock is favorized by the straight handle as it´s easier to force the position of your wrist in this direction (IMHO).
The straight handle makes possible extreme positions to maximize the power of different strokes. This is made evident in nowadays, especially, in the backhand of several top players. To me one of the most noticeable example is the Ovtcharov´s backhand:
These arguments might help to explain why many offensive top world players are currently using straight handles, the list is long but among the more relevant ones you can find: Jan Ove Waldner, Timo Boll, Dimitrij Ovtcharov, Patrick Franziska …
Wise Table Tennis 
Bernard Hock insisted that only straight handles worked. Interesting, though, that the above are all Europeans and they’re all men. What do Ma Long, Zhang Jike and Liu Shiwen use?
Chinese players using the shakehand grip use the flared handle. I do not remember any of them that use straight handles. I imagine that there has to be very good reasons for that. Among woman (chinese or not), apart from defenders, I do not know any of them that use the straight handle neither. Perhaps someone might put examples of females using the straight shaped handle?