Ftw, I ran some numbers awhile ago looking at what the debt would look like after grad. Assuming you get hit with the max loans, you'll end up with ~$300k in debt when you graduate. If you choose to pay back the debt over 10 years so you're eligible for LIPP, you have two 'worst case' financial scenarios*.
1. You take a public interest job and only make $40-50k per year. Thanks to LIPP, you'll pay next to nothing for your loan, and Harvard ends up paying for your education. You aren't making a lot of money (at least for a lawyer), but you're hopefully doing something you love and, at the end of the day, $40k per year isn't a terrible income. Plenty of people are able to make that work for years, and if you're happy, it's probably worth it.
2. You take a higher paying job (clerkship, gov work, etc) and make $60-100k per year. While you're eligible for LIPP, you pay for a good share of the loans every year, but still take home $40-60k per year. At the end of the 10 years, you've paid about $300k, aka the cost of a Harvard degree. HLS isn't giving you a discount, but it is allowing you to pay for the degree when you actually have the money. More importantly, you're still bringing home $40-60k, which might be frustrating, but it's still a good salary.
In both cases, your worst case is being a member of the middle class for 10 years, after which you can really start to save and move up if you want. For me, that's pretty much what I would be doing if I didn't get a law degree. The big difference is that I think I'll have much stronger job security with a law degree, it will allow me to work in a more compelling field, and if I really want, I can take a job that pays much better so that the loans aren't an issue.
*I suppose the real worst case is not getting a job when you graduate. Only 14 of the 2012 grads couldn't find work, but I know that won't be an issue for me (I know enough lawyers in my hometown that I'm sure I could find a gig even if I don't want to stay there long term).