Rimmel London Extra Super Lash mascara is a good volume building product, at a very affordable price. It's an adjustable product, in that the more coats you apply, the more it builds up your lashes. A few quick strokes of the product and you'll have nicely separated and defined lashes. Keep going and your lashes will get darker, thicker, and more dramatic with each application.
For those with sensitive eyes, lash-builders have a tendency to irritate. They also smudge and clump. Rimmel goes on easy, without irritation, and lasts a good 8 hours. At the end of the evening it may get a bit smudgy, but much less than other lash-builders I've tried. It also washes off easily with soap and water, something many mascaras promise but don't deliver.
As the product ages, it gets a bit more clumpy, so it's a good idea to replace every couple months or so. This is the standard for eye make-up anyway--it's best for eye health to replace products on a regular basis.
There are more expensive products on the market that can probably get you even bigger lashes, but Rimmel's Extra Super Lash is a worthy alternative. At just around $3, it's a bargain for providing nicely built lashes, a super dramatic black color, and a long-lasting product with little to no irritation.