If I read their site correctly, you get your paid principle back, plus 25% of the appraised value appreciation.
Example: $100k house, you put down $5k, they put down $15k.
Say you live there 9 years, the house goes up to $120k in value, and you've paid off $12k of principle, and you're ready to sell to move somewhere else. They get first dibs to buy the house, and their price will be $90k - paying off the remaining $68k of mortgage, your $5k down payment, your $12k principal, and giving you $5k of the appreciation. They keep the $30k difference (part of which is paying back their down payment).
Compared to putting 20% down yourself, the advantage is you don't have to put up that extra $15k. Compared to putting 5% down on a mortgage that allows it, your payments will be lower because the mortgage balance will still only be 80% to start, and no PMI.
The disadvantage is you only get $5k of the appreciation, instead of all $20k.
It doesn't seem like a good deal to me, especially if you still have to pay transaction fees (closing costs, realtor fees), since you'll have much less possible appreciation to offset them. It may still be worth it if renting is quite expensive compared to buying, and it would take you a long time to save up the down payment, but definitely run the numbers.