I started buying the moderate portfolio, $100/month. I'm at $610 now (after 6 months), but you can't really tell much based on short term performance. It's pretty easy to track though, they update the fund prices on their site everyday, so I can log in and see what my funds are worth at any time.
From my experience so far, the bond funds only move around a penny or two a day, so get moderate or aggressive if you're not strapped for cash.
It's also handy because you can have them automatically take the money from your ING ISA.
So I've been reading around and it looks like there are no fees for ING Direct Funds for buying/selling. Are there other mutual fund related fees you have to pay though?