Myself, over 2004, I downloaded one track and was not impressed. I decided to try out a few sites today and have come to the conclusion that there are essentially four reasons why the current model sucks.
1. Compression. Most legitimate sites supply songs in Microsoft's WMA format for Digital Rights Management Purposes (almost all in the UK use OD2 as a backend as well). Those with competing hardware (eg Apple, Sony) or software (eg Real) use their own propriety formats with similar restrictions. To save on bandwidth, all of these use what's known as lossy compression, which leaves out “unheard” parts of the sound in order to reduce the size of the files by large amounts. Even those rare sites that don't supply rights-crippled tracks provide MP3 or, very, very rarely Ogg, both lossy compression formats. This isn't, in itself, entirely bad, but what happens when a better compression system comes along? For example, the MP3 format was first proposed 15 years ago and finalised five years later. Ogg, a much more recent development, can produce either smaller files with the same sound quality or better quality in the same file size as MP3. The Ogg compression though leaves out a difference part of the sound than MP3, so converting between the two formats is not recommended. Having compressed audio as your source means you're stuck with it, even when it's superseded.
2. Price. Tracks in the UK go for 69p on the OD2 sites and albums pop for £6.99. This produces some odd effects... for example, the MSN Music store sells the 5-track Genesis Live album from the mid-70s for £6.99, but if you bought the five tracks seperately, it would only cost you £3.45. On the same group's Foxtrot album, the one minute forty seconds of Horizons costs the same to buy as the twenty-three minute-long Supper's Ready and the entire album, bought seperately, would cost only £4.14. However, should you think the 18-track Best of Blur would be a good deal, you'll find it's not available as a single purchase and you'll need to spring for £12.42 to get it. The Complete Animals meanwhile, a hefty 39 tracks, will set you back £26.91 in individual purchases. Meanwhile, on Amazon.co.uk, Genesis Live, Foxtrot and the Best of Blur all cost £5.97, whilst The Complete Animals sets you back a whole £9.99.
3. DRM. I mentioned it in the compression bitch, but it's worth repeating: most tracks are rights-crippled. The limits that can be set include how many times a track can be burnt to CD, moved to a portable device or transferred to a second PC. Some of these seem ludicrously low - the Scissor Sisters album can be transferred three times and burnt once. Course, once you've burnt the CD you can rip it yourself and have DRM-free tracks to use, but this just makes the protection even more of a pointless hurdle.
4. Fuss. Apple, Napster and Real all require you to install their software to even browse their shop; then Real doesn't even have a UK shop. Sony's connect allows you to browse the shop if you use IE, but requires software to buy and “manage” your downloads. All the OD2-based shops also require IE and, for the MSN shop at least, some cheap ActiveX downloader. Tesco's downloads does work in Firefox, but whoever is supplying their backend is down. The only decent, well-thought out shops I've found are Audio Lunchbox (which sells MP3 and OGG formats) and Bleep (just MP3s currently, with OGG and FLAC being considered) - unfortunately, they have a limited range of tracks (some interesting stuff though) and Bleep's interface is pants if you don't know what you're there for.
I can't see myself using these services much as they currently operate. If they sold FLAC format stuff at these prices, I'd be there. But not until then.