As far as I'm aware, this is the best repayment option you can get, which is IMO, pretty sad. As always, feel free to prove me wrong--I'd like to know if there are more/better options. You are right that you can get more paid off (75% instead of 50%) if you work at the NIH for 3 years as opposed to the max of 2 elsewhere. Again, this caps at $70k (or $105k at the NIH), which is nothing compared to ridiculous sums like $250k that many med school graduates face.
http://www.lrp.nih.gov/about/extramural/particip.htm
In exchange for a two-year (or three-year for the Intramural General Research LRP) commitment to your research career, the NIH will repay your educational loan debt at the rate of 25% annually, up to a maximum of $35,000 per year. You will receive the maximum annual disbursement only if your repayable debt remains equal to or greater than $140,000 over your LRP contract. The NIH Loan Repayment Programs (LRP) payment projections are based on the repayable debt calculated at the start of an LRP contract. Repayable debt is a participant's total eligible student loan debt (principal plus existing accrued interest) at the beginning of an LRP contract, less participant obligation (which is 10 percent of a participant's base institutional salary), plus estimated future interest on a participant's eligible loans through the end of the LRP contract.