I think if your need it, you've got problems. Basically, higher octane fuel is simply more resistant to detonation or knock. You get this beause something the engine runs so hot that something is igniting the air/fuel mixture, besides the spark plug, before it's ready (like when the piston is still coming up). This is highly damaging to an engine because it's simply not designed to handle stresses like that. If your daily driving a car and having knock issue that could be helped with something like an octane booster, your just bandaiding the problem rather than cutting back the boost, backing off the timing or simply running the engine a little richer (assuming you have a way to do that).

That said, there are some modern cars that might benefit from that, especially if the highest octane you can buy from the pump is 91 (I can get 94 here in PA). A car with an adaptive engine managment that's watching the O2 sensor and knock sensor results might dial in a little more timing if the fuel will support it, giving you a little more power. Also, a lot of high speed, high tempurature driving might benefit from the additional knock protection, but again, it's probably better to address those issue rather than trying a magic juice to solve them.

Long story short, probably no major downside, aside from cost, for some additional protection, but not a solution or alternative to proper tuning. I've actually always wondered how those work? I'm assuming they can't sell simply high octane race fuel in a parts store bottle to dilute and raise the octane in your tank a little. A modern fuel pump has all the chemicals and addatives your engine needs so it may just be snake oil?