I have one of those in the mail. I've been using ToToTek's products for years, and I've always been pleased with their converters. The last one I bought before this was a PlayStation to Neo Geo converter, and it works flawlessly. I had to get this, because diagonals are so stiff, they're almost impossible to hit on my TG16 controllers. I was going for a high score in Soldier Blade's 2 minute caravan mode, but without smooth diagonal movement, I'l never go higher than 10th place in the Shmups ranking.
tl;dr - Get one of those converters today.