One thing I miss a little is that ack has the super convenient:

    ack --java "foo"

while with ag you write:

    ag -G"\.java$" "foo"

But yes, ack and ag feel pretty identical except for the speed. Most of the time the speed improvement is irrelevant to me, except sometimes now I'll use ag in my home folder, and it's still fairly snappy.

ag is not "exactly like" ack. There are features that ack has that ag has chosen not to replicate. ack is also more portable.

That's not a knock on ag at all, and if ag fits your needs, then by all means use it.

I normally tell people to use ack because it's like grep but faster (owing to it's sensible defaults) ... if I use this I'm worried I might go too fast and travel backwards in time or something.