Yes, they work! I've tried Revitlash and MD Lash Factor. Both make your eyelashes longer, but I did notice more fullness with MD Lash Factor. My preference is definitely MD Lash Factor. It will give you length, fullness and it will provide that natural curl so you won't have to use a lash curler. All I needed with MD Lash Factor was mascara.
Both lash conditioners will give you the length that it claims provide, the downside....you still have to use that mascara to make them stand out.
When I used MD Lash Factor it took about 2 months to notice a difference in length and by 3 months I didn't have to use a lash curler...just mascara.
3 of my clients are currently using the product and they love the results.
Lash conditioners are great in conjunction with lash extensions. It give the lash stylist a great foundation to work with, but it's always good to check the ingredients to make sure it does not conflict with lash extension adhesive.
So....my product of choice for lash conditioners is MD Lash Factor. http://www.mdlashfactor.com/