I've been playing TF2 as a premium player even before Chemistry Sets came out, and I literally haven't found a single one. Are they even rarer than hats, tags, paint, and the works?
Chemistry Sets are indeed a fairly rare drop. Keep in mind there are several different chemistry sets as well:
- Strangifier chemistry sets: Prices range widely depending on the item. There were some chemistry sets that stopped dropping some time ago, which have grown to be fairly expensive. Other strangifier chemistry sets, like for the Professor Speks, are pennies on the dollar.
- Collector chemistry sets: These are significantly more rare than strangifier sets. If you are lucky and find one of these, you can easily sell it on the market for 8-10 USD (some even go upwards of 50 USD).
If you're wanting to work towards completing a chemistry set, it would probably be easiest to simply trade for one that you want. The likelihood of a desirable one dropping for you is incredibly slim, and most likely not worth the time to wait for one.