investment in a ponzi scheme
It's not completely inconceivable.
If I understand correctly, now after 15 years, you receive 50% special interest on the current years savings? So about 3.3% per annum on the entire balance (less if the interest is rolled up into the balance).
I would be sceptical if it has any real value, unless you're talking about large sums. The reason I say that is 2 fold:
1. Although there's no end date, it'd be unusual not to have some over riding clause that would allow the bank to terminate the product, and
2. It's already past the highest rate, so there's a decreasing annual return on the balance from here. Essentially you're asking how to value now, a guaranteed 2.5% return per annum 5 years from now.
Assuming subject to DIRT, you're comparing with tax free returns on state savings. OK, it's a very decent return in current market, but put lump sum into state savings, and regular monthly into best regular saver, and it's not earth shattering by comparison.