Logitech S715i

Pros: not too big, quite sturdy and well portable
overall good and full bodied sound when plugged in

Cons: sound quality and max. volume drop significantly when run from battery
when played at maximum volume it turnes off after 30 minutes

Summary: the s715i seems like a poor man's bose sounddock portable. indeed when plugged in the sound comes quite close while costing a fraction of the bose. the s715 has a nice deep bass, pretty impressive for the size, while being even quite a bit smaller than the bose. treble is quite dominant but it is lacking mids a bit.
unfortunately the whole sound-magic disappears when run from battery. bass gets cut by half and maximum volume is reduced as well. you won't be able to rock a barbecue-party with the s715i when run from battery. with a docked iphone maximum volume is maybe suitable for a small room. but with other players (in my case cowon M3 running rockbox) attached through aux-in i was able to achieve a much higher maximum volume. i could have turned even louder, but the s715i amplifier already started to clip. with a good mp3-player you can also try to add a bit of bass-boost, don't try this with an ipod as their EQ distorts depressingly.

i also noticed that when played near maximum volume the s715i tends to turn off automatically after half an hour. when i reduced volume slightly it still continued to play for a while, but started to shut off again later. the battery seems to be the weak point here. mine is still brand new, but i have read from others that the battery won't hold charge after some days.

logitech's s715i would have been a killer speaker, if they managed to keep the great sound when run from battery too. for portable use i still prefer my bose sounddock portable, as it sound so much deeper and louder than the logitech.
but in reality there are hardly any alternatives with this size and price that sound better, so the logitech remains a kind of king in its class, although it could have been so much better.