TIVO hardware has been a bit of a disaster in the UK, the whole reason they are turning up dirt cheap is that the manufacturer and retailers are dumping stock. There is no sign of a second generation unit being released in the UK even and even in the US only 450,000 units have been sold (according to a TIVo fianancial report). However with Sony and Toshiba buying into the technology it should appear in other recording devices so the technology will survive just not in it's present form.
TIVO software will be around for a long time though, the subscriptions both monthly and one off should enable the listings services to remain profitable and any new hardware will need that service as it's the software that makes TIVO special not the hardware.
So if you have TIVO now I don't believe you have any worries and if you can pick a cheap one up then go ahead and if you pay the one off subscription with certain credit cards and they do go bust you can claim back the money (So I've read).