In some senses, any answer to this question is going to be opinion based - nobody outside of HFT firms really know what they do, as they tend to be highly secretive due to the competitive nature of the activity they're engaged in. What's more, people working at HFT firms are bound by confidentiality agreements, so even those in the industry have no idea how other firms operate. And finally, there tend to be very, very few people at each firm who have any kind of overall picture of how things work. The hardware and software that is used to implement HFT is 'modular', and a developer will work on a single component, having no idea how it fits into a bigger machine (a programmer, for instance, might right routines to perform a function for variable 'k', but have absolutely no idea for what 'k' stands!)
Keeping this in mind and returning to the question . . . The one thing that is well known about HFT is that it is done at incredibly high speeds, making very small profits many thousands of times per day. Activities are typically associated with market making and 'scalping' which profits from or within the bid-ask spread.
Where does all this leave us?
At worst, the average investor might get clipped for a few cents per round trip in a stock. Given that investing buy its very nature involves long holding periods and (hopefully) large gains, the dangers associated with the activities of HFT are negligible for the average trader, and can be considered no more than a slight markup in execution costs.
A whole other area not really touched upon in the answers above is the endemic instability that HFT can bring to entire financial markets. HFT is associated with the provision of liquidity, and yet this liquidity can vanish very suddenly at times of market stress as the HFT remove themselves from the market; the possibility of lack of liquidity is probably the biggest market-wide danger that may arise from HFT operations.