The tracks downloaded with Comes with Music will only work on the Comes with Music handset and on your PC. In the case of the £82 5310 it does rather lend itself to a PC only option - you can eBay the phone for around £60 which means, in effect, you can get unlimited downloads for your PC for about £30 for a year. Given you keep the tracks even after the subscription expires that's potentially very good value. It's probably not what Nokia has in mind, but even so... Of course opting to keep the 5310 is a viable option too; it's a decent back up phone after all.
It is still early days for Comes with Music, but, thus far, is seems to have failed to capture the imagination of consumers. One issue seems to be explaining the proposition to people; because its a fairly radical departure from previous offerings there is an element of its too 'good to be true'. Explaining the offer to the typical feature phone buyer, where service usage is still relatively low, is not a straight forward proposition. Clearly DRM and licensing issues will be a concern for some, but this is more likely to prevalent amongst higher end users than those looking to buy a handset like the 5310.
A more significant concern is lack of availability in terms of both distribution and handsets. Currently Comes with Music is only available in the UK on a small number of handsets through limited distribution channels. I think it unlikely that many consumers will go out of the way to buy the service (i.e. they will not change where and how they buy a handset, but, if offered the option, would be interested).
Outside the 5310 (only available through Carphone Warehouse and the Nokia store there is the Nokia N95 8GB, which can be bought on a contract through Carphone Warehouse or direct from UK operator 3. Nokia did recently add the Nokia N96 to the Comes with Music line up, but it is currently only available SIM-free from the Nokia store. Until we see Comes with Music on more operators and on more handsets then its uptake is going to be limited.