I have very sparse, fine, short ones and they've recently become a bit more numerous due to diligent use of lash conditioners, etc. I use Mavala Double Lash at night in conjunction with jojoba oil and Avon Beyond Color Triple Benefit Lash Conditioner since the latter is too obvious on my lashes to use during the day. I use this during the day under my mascara (which is waterproof--bad for lashes). The Ardell alone probably will not lead to any noticeable growth, but works fine to lightly condition eyelashes and maybe provide some sort of barrier between them and the mascara. It might work as a kind of clear mascara for people with thicker, lusher eyelashes than mine, but I wouldn't count on it for that. It's very cheap (around 3 USD at the drugstore), comes in a tube with a mascara wand applicator (a big, bushy one which does a better job at coating lashes evenly than the Avon or Mavala, BTW). There's a slight, glue-y scent, but it's not strong or bothersome. I've not noticed any weird side effects from using this (I wear contact lenses) and it doesn't de-curl my lashes or mess up my mascara. May be worth a try if you're in the market for a cheap lash conditioner.
ETA: This does hold a curl pretty well, but if you have fine, fine lashes like me, then you need to brush this on first, allow it to dry, then curl. Seems to make mascara go on smoother and I like using something that's supposed to help my lashes beneath something that kinda hurts them. Maybe it cancels out?