As we always say around here, you'll never know until you try it yourself, but based on some listening I did at the dealer, I think the M5 can be a suitable replacement for the BS1 in some scenarios... assuming you don't need portability, and you like the looks.  

I was looking at the M5, BS1 or BS2 for my office.  I went with the BS2.  I know that it's not a fair comparison, but I was also factoring in price and the fact that the majority of my listening is at low levels, and I'm working so I'm not exactly paying close attention.  My ranking was BS2 (mk1, dealer ex-demo price), M5 and BS1.  Based on my BS2 performance, I think the Bs1 gets slightly higher marks over the M5 for the 360 sound, but again, that may or may not matter to you.  It didn't matter a lot to me.

I believe there's a BS1 review on Amazon where the reviewer tried the BS1 and went back to the M5 (although there may have been problem with the Bs1 so maybe it's not totally relevant).

Then again, if you buy a BS1 before the end of the year, you can get a M3 for free - but that might be a similar cost as a M5 + M3.... 

Frankly, just the aesthethics of BS1 vs M5 are a decisive factor IF you can afford it. 

The BS1 is a 2 way speaker with its tweeter on top. 

The M5 is a 3 way with a imho better sounding midrange. 

The BS1 has the advantage of being portable and taken outside on the terrace. 

Difficult to choose. But the M5 plays very well every day for a year now.

I have both BS1 and M5 and after reading this thread I thought of doing a very simple side by side listening test. Even though I owned both since their respective launches, I have never compared them side by side.

Setup:

Placed both speakers on a pretty solid table in my living room. Both speakers where set to free standing with loudness activated. BS1 had bass and trebel set to "0". My livingroom have hard surfaces so I think there are quite a lot of reflections.

I placed myself on a chair about 3 meters from the speakers. I did not move around to evaluate the 360 capabilities.

Listened to the following tracks from Deezer HIFI:

• The Hunter - Jennifer Warnes
• Money for nothing - Dire Straits
• Skulls - Röyksopp
• Bird on a wire - Jennifer Warnes
• Embracable You - Lisa Lovbrand

Result:

Bass: M5 a clear winner. I experienced the M5 less "boomy" while listening like this. On lower volumes in a smaller room the bass is a bit to much. BS1 bass is not bad but lacks in comparison to the M5. Think BS1 benefit from a placement near a wall and you can tweak the bass and treble individually if needed.

Midrange: Generally the M5 had a more pleasant sound in comparison. BS1 sounded more like a "can". Probably the BS1 is suffering from not having a dedicated midrange speaker. Instruments like the saxophone and piano in the beginning of the track "Embracable You" had good clarity on the BS1 but when the voice came in I could hear something was lacking. 

I had the impression that BS1 was giving a bit wider soundstage heard from my seated position. Music from M5 came more from one spot. I bit hard to describe...

Verdict:

So in summary, I think M5 actually are the winner when it comes to my listening test.

Old thread but since I have both the BS1 and BP M5, I'd throw in my 2 cents worth. The BS1 easily wins on aesthetics and portability (though you have to buy the optional dock for easy portability). The battery life is excellent, it looks amazing and it is very powerful for it's size. My version also has Google Voice that works really well. 

The BP M5, however, to me sound better for the music. Not by much, but noticeable when played side by side. The M5 is a really good value.