The eagerly awaited music-streaming service Spotify has today arrived on both the iPhone App Store and on Android’s Marketplace, and I’ve been granted a seven-day guest pass to see if it’s really worth that £10-a-month premium account.

Over the course of a year that does seem like a lot of money – particularly as most users will already have huge music collections of their own – but the promise of millions of tracks available on the move is certainly tempting.

Starting up

Once logged in, you’ll be delighted to see all of your desktop playlists seamlessly synced with Spotify on your phone, and if that’s what you’re after you can just dive straight in.

Over Wi-Fi, tracks play immediately; on 3G we had pauses of anything up to ten seconds, and EDGE is even longer. But once the track is going we were only scuppered by the occasional drop-out for a few seconds – and our office is a notorious O2 blackspot.

When tracks are playing the screen looks similar to that of standard iPhone playback, with the usual controls, a CD cover if Spotify can find it, and an extra options button in the top-right corner.

Tap this for details of the album and artist, an option to add it to an existing playlist, and extra shuffle and repeat controls.

Offline playlists

Altering playlists on your desktop client brings a seamless update on the iPhone app too, in mere seconds if you’re on Wi-Fi. And you have the incredible option of turning them all into “Offline playlists”: next time you connect to Wi-Fi the entire playlist will download onto the phone for use at any time.

The limit is a rather odd 3,333 tracks, and all you have to do is connect once every 30 days to prove you’re still a Premium subscriber. That’s a pretty phenomenal amount of music at your fingertips, and conveniently sidesteps those obvious issues while on the tube or out in the wilderness.

Searching is simple enough, with three tabs to arrange results by Tracks, Albums and Artists. It’s not something you’ll want to do regularly with the iPhone keyboard – you’ll create a playlist in a fraction of the time using the desktop client – but it works well for finding that track you just heard in the pub.

That’s the real meat of the app, and the only other addition is a More… button on the homepage, with details of the app, a link to Spotify.com’s help page, and an option for forcing offline mode even in network areas. And that’s your lot. It all works remarkably smoothly and won’t be at all unfamiliar to users of the desktop app.

The caveats

There are a few key issues though. The first is Wi-Fi, which works fine in most cases but currently refuses to work over firewalled networks. So, no office Spotify just yet, and it also casts doubt over the app’s usefulness on public Wi-Fi networks. Spotify representatives have suggested opening port 4070, but in Starbucks that won’t be an option. Rest assured it’s high on the list of fixes being worked on.

Then there’s the iPhone’s annoying single-task way of working. The functionally identical Android app can play tracks in the background while you work, but on the iPhone you have to have Spotify open at all times. Nip out for a second to check your emails and playback will stop dead, although it will continue from that point when you next load it back up.

And finally there’s the issue of battery life. The iPhone’s music player isn’t exactly kind to the battery anyway, but when you’re adding data transfer on top of that too, it becomes greedy to the extreme. A mere half-hour of streaming over 3G has taken a good 25% of my battery bar (on a last-generation iPhone 3G), so I wouldn’t be too confident of using it for the duration of a long train journey. There’s not a great deal Spotify can do about it – although the offline playlist option should reduce the effect by quite a bit.

These are certainly drawbacks, and they must be considered when the cost is a significant £10 a month. But all in all it’s a hugely impressive debut. A few features of the desktop client haven’t made it across – the radio channels and top lists, for example – but as a standalone player for specifically selected tracks and playlists it’s a very tempting proposition.

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UPDATE: The battery issue doesn’t seem as bad as I initially thought. Now the setup is done and I have less need to have the screen on and draining the juice, I managed a good half-hour this morning with only a minor dent in the battery bar.

A far greater issue has arisen, though. On a short 20-minute walk from Charing Cross to Soho, my 3G connection dropped out no fewer than six times, each time for around 15 seconds. One of those six times the Spotify app was even kind enough to close itself automatically. It’s annoying enough to put me off, so I guess the offline mode is going to be the major way I’ll be using Spotify from now on.