For everything there is a season, and a time to every purpose under heaven.
There is a time for both types, and anytime we preach we should consider what is best for the time, opportunity, subject,audience etc.. that we are given, when I do preach topically, I try to exposit as much as possible.
I remember listening to John MacArthur"s radio show when he was teaching on expository preaching, and he was talking about the importance of using your time wisely and the realization that none of us will probably have the time to exposit the entire Bible, he told about a pastor, I can't recall the name but, the pastor started his pastorate preaching at Isaiah 1:1 and 40 years later when he retired, he had just gotten to Isaiah chap 26.
The one pitfall I would warn those that are bent toward the topical style from falling into is don't come up with a sermon in search of a scripture, I've heard too many topical preachers start with a verse, and by the time they finish, they are miles away from that verse and no one remembers how they got there.