I have an SSD in my late '07 Blackbook. It made a night and day difference.
It's a shame the "classic" Macbook now shares a forum with the new Macbook, as before that computer came out this forum was one of the few "safe havens" where the upgrade-itis that's everywhere else on Macrumors was nicely absent. Unfortunately, this forum is now saturated with the knee-jerk "Buy a new computer" people.
Regardless of the bus speed, SSDs still make a big difference in performance. They will generally saturate the bus under most conditions, where a platter drive will not. I've found this to be true even with the ATA/100 bus on Powerbooks.
A new Mac is going to be $1K+ while an SSD-depending on capacity-will generally be under $100 in the sizes many folks will put in Macbooks. Even a high end SSD, like the 1TB 850 Evo I put in my 2012 Macbook Pro a few weeks ago, cost me $330. It's an upgrade that will milk a few more years out of the computer, especially since at least for the time being 10.7 is reasonably current. IMO, there's no reason not to do it, especially if a new machine isn't in the budget.