Deet alone is useless against gnats, no-eee-ums and those nasty biting black flies! Here, it has to be spiked with an additional repellent to be effective.
For DEET products, I've got a lot of performance from Sawyer products. I liked their 20% timed-release repellent that was every bit as effective as the 100% for up to 5 or 6 hours. Also, Sawyer's "Broad Spectrum" - DEET spiked with another chemical - I found to be very effective against those biting black flies.
Dale