If I have serious erasing needs, I'm going to reach for a dedicated eraser. But most of the time I have a pencil in my hand, I only need to erase a few numbers or am away from my desk. I don't want to carry around a serious eraser--this is where having a built in eraser is really nice. The obvious solution is the eraser cap. ANY pencil can have its own eraser. You can have the best of both worlds with an eraser on your Tombow, Caran D' Ache, Mitsubishi, Staedtler, or vintage pencil.
Utility aside, pencil cap erasers don't get much respect. Having an extra large replacement eraser on your pencil makes you look childish, nerdy, or like someone who makes lots of mistakes. On a strictly aesthetic level, the cap eraser is a blight on the otherwise sleek form of the pencil. A Tombow Mono 100 with a cap eraser is like a Japanese businessman wearing a mullet wig. That being said I will admit that I don't always need an elegant pencil, and hey, mullets are ok on some pencils. When you're not concerned about your appearance and need a purely functional pencil, go ahead and throw on a cap eraser and commence to erase furiously.
There are surprisingly few name-brand cap erasers on the market. Unbranded cap erasers are tempting, but they have a high probability of tearing or breaking. Two commonly available quality erasers are the PaperMate Arrowhead and the Pentel Hi-Polymer. Quality still comes cheap when you buy a pack: the PaperMates come in at $0.03 each and Pentels at $0.20 each.
This is the cliche pink cap eraser. The walls are extra thick, the eraser portions are fat, and it fits snugly on the end of the pencil. The rubber itself is fairly stiff, so it feels very solid when you erase. This cap eraser has excellent performance, but the large stubby shape and bright pink color make it stand out like a sore thumb. I think they look their best on a yellow pencil, but use at your own discretion.
Pentel's eraser formulation is high quality. This eraser has a much sleeker shape, and the white color is understated in comparison to the PaperMate. While it looks a lot classier, the eraser feels a little flimsy as a consequence. I think they actually look ok on black pencils...almost not embarrassing. The Pentel erases about the same as the PaperMate, but doesn't feel as nice to use.
So there you have it... Cap erasers are great to use, but they are the pencil equivalent of wearing a rubber nubbed finger tip protector. When you put function ahead of form, use a PaperMate. If you still want the benefits of a cap eraser but want to look a little more discriminating, choose the Pentel. If you are too cool to use a cap eraser at all, try a small eraser like the General's All Art.