High-frequency traders (HFTs) have received a mixed reaction from academics and practitioners with some people underlining their role as liquidity providers and others highlighting the problems that they could bring to the market.
From summary: Do HFTs exploit their small (milliseconds) latency advantages to anticipate orders arriving in very quick succession at different trading venues from other market participants?. The regulatory EU set-up makes it more difficult to predict where orders will be routed, compared to the US market?