I have no idea what is wrong with Mail or why you wouldn't recommend someone use it.
Mail was Steve Jobs' baby ever since NeXT, and it was built to be used (and useful) internally at NeXT/Apple. It's the best mail client I've ever used, in a sea of extremely poor mail clients.
On my Macbook pro:
Mail is horribly slow with my 5gb of Gmail. And searching in Mail is even slower and worse, unreliable.
I have had better luck with Sparrow, but even that has its bouts (its slows down all of a sudden and then comes back, which makes me suspect its doing some background compaction/garbage collection).
Even though I am no longer a huge fan of the Gmail web interface, it still works the fastest.
Mail was Steve Jobs' baby ever since NeXT, and it was built to be used (and useful) internally at NeXT/Apple. It's the best mail client I've ever used, in a sea of extremely poor mail clients.