I love 99's. I think they're adorable, like a puppy version of the 66. (And who doesn't love puppies?) I have two 99K's from the mid-50's and I also have a 185J, but can't remember age on that one. They all are great stitchers.
I have to confess I don't use them very often, though. I'm always sewing on my "newest" machine in the herd, which lately I've been into zigzaggers, I have a Universal I'm toying around with and still buffing up "Betty" the White.
It's interesting comparing my two 99's - they're only a couple years apart in age but they run pretty differently. One I bought at a thrift shop and it looks like it just came from the show room - doesn't look like it was used at all. The other was abandoned in a garage and given to me for free; wiring was shot, lamp was gone, and she's definitely seen a lot of use. I bolted on a different motor that had good wiring (and the lamp intact) and that 99 runs so much smoother than the "new" one. Neither runs poorly, but the one that's been used runs smoother - with the belt off that thing will glide for a LONG time; the newer one glides for a bit but stops sooner, even with fresh oil on both.
I think that's neat because it so clearly illustrates the difference between a broken in machine and one that's still "new".