I have a T61p and because I travel a lot by train, I have 2 batteries. The original 9-cell bought with a laptop + a replacement that I just lost after 14 months of use. Usually, while driving, I discharge both batteries, and I have such journeys on average 1.5 times a week. The original one is already 3 years old and keeps quite quite. I think it has retained over 50% of its capacity. She survived two substitutes. It can be said that if instead of the first replacement, I had bought an original, non-aging one, I would have gone to zero

oh yes, I'm sitting around looking for forums and I'm wondering whether to buy a new replacement or a new laptop ;)
Before that, I had a Lenovo N100 3000 and had a total of 4 batteries there. All original. The first one with a 6-cell laptop died after a few months and they replaced it with a new one under warranty. It was a whole defective series of these batteries, and they even replaced them for free with those that had not yet died. The third battery was 9 cell and held very well, but after two years its capacity had dropped more than my current T61p. Then I bought a new 9-cell for it, also original, but I didn't use it for a long time because I had an opportunity to buy the current T61p so the N100 was sold to a friend. As far as I know, she is not satisfied with these batteries.
Also to sum up from my experience with batteries: a good replacement has a price that is adequate to the quality. Ie 3x cheaper for 3x shorter life time (I wonder what our natural environment says about it). However, buying the original, unfortunately, does not guarantee quality.