Before I bought a Micro I wondered for a long time whether it would be worth it, since I already had an SP. Now I have both, so I can answer that question for others.
It was worth it.
For me, the size and weight have made a massive difference to the way I use it. Unlike the SP, the Micro fits in my pocket with my mobile phone comfortably. I only ever took my SP out with me if I was sure I'd play it. But now, at the start of every day I pick up my mobile and my Micro together when I leave the house. 90% of days I won't play it at all, but whenever I want it it's there. On the days I don't play it I don't actually think about it or notice it's presence.
Secondly, the Micro has reinvigorated my interest in GBA games. Every game looks better on the Micro's screen. Old GBA games now look like new technology, and I think I'm going to be buying new cartridges for a lot longer than if I just stuck with my SP.
A lot of talk praise has been made of the PSP's screen. Obviously the Micro's is much smaller, but in my opinion it's also sharper and definitely brighter. The Micro's screen doesn't get greasy and show up finger prints in the way the PSP's does.
It's because it's so much sharper that you don't need to hold your face as close to the screen as you might initially expect. Slightly closer is more comfortable though.
The Micro has a headphone socket. Enough said there I think! ;o)
The battery seems to last as long as the SP's and the buttons no better or worse or less comfortable, just different. I got used to them very quickly.
If you consider yourself a GBA fan, and if there are still GBA games that you never got round to playing but would be interested in, I would suggest you get a Micro. One random day when you're bored you may find one of those games in your pocket, and it will look and feel the best that it can.