I mainly wear dresses with pockets (White Stuff and Joules are good for these) where possible, I cut a little hole in the pocket and feed the tubing through. I have the 640G too and it is bulky, but do-able. I have put it in my bra on occasions I'm wearing a dress without pockets. I've got a SpiBelt-type band from TK Maxx that I use when running and I put this under my bra for the xmas party, had to wear a push up bra as my cleavage isn't very big, but it worked. You can program the meter with preset dual waves or do a manual bolus, and so I guessed in advance that I'd eat 100g carbs and set a 50:50 dual wave over an hour so I could deliver it remotely. The carbs were about right but then with a bit of dancing I ended up hypo, but that would have happened with using the pump directly too - a learning point for next year!
I've also got a couple of dresses with bat-wing type sleeves and I can put the pump (without the clip) in the side of my bra and use the remote functions, it's not too uncomfortable and you get used to it. When I went to choose a pump at the clinic I actually borrowed them all and went to the loos to try stuffing them down my bra and see how discreet it could be. The Insight did appeal because it is smaller, and has the full remote functions, but I don't really like touch screens and the remote seemed very slow to respond, so I choose the 640G because it seemed to have more pros than cons for me, but everyone is different, so make sure you get to handle a pump and try it for size if you're getting one.