If you can afford to it's a good idea to have a spread of investments for retirement - pension, savings and property for example. The key part of that however is 'if you can afford to', which a large majority cannot if they want to actually have a decent existence for the bulk of their lives. Doing so however, makes you liable to have to pay for some or all of your care costs if you become unable to look after yourself.
An ISA pension would have to have an interest rate that was at least equal to or slightly higher than inflation (and I'd personally go for RPI inflation not CPI). It was also need to be ring-fenced so that the government or financial institutions could put the capital at risk by dodgy investments or someone going bust. Such products are indeed available, but again, are normally only feasible if you have a fairly large lump sum to invest to start with (£10k + typically).
I must confess, another aspect of the mandatory pensions situation that concerns me is that employers will be tempted, soon or later, to only offer the statutory minimum pension rather than any better current schemes they offer. Why pay in 5, 7, 10% of your employes salary when you can get away with 3%?
Another issue with pensions is that we've seen repeated examples of governments seeing pensions as some sort of emergency fund that they (or business) can raid periodically either through changes to the tax system or making it favourable for companies to have pension payment holidays.