I found when DS1 was born i needed a bigger nappy bag to fit everything in- cause of all the just in case situations. The bag i got actually had a small bag attached on the front which could be unzipped off and used separately. When DS1 was about 17months i decided to switch from disposable nappies to MCN's- so still needed the large bag for longer trips - i would just use the small one for quick trips out.

Then when DS2 arrived i found i needed an even bigger bag- especially as i had 2 in MCN's. But then a few months later i went back to previous larger bag (ended up getting a new one about now) as not as many nappy's were required for DS2 and DS1 had begun toilet training.

Now my boys are 17 months and 3 years old i usually just take the smaller bag out- which i have 1 nappy, spare undies & pants for DS1 and the usual essentials like tissues and wipes. I then give DS1 his favourite little backpack which i put 2 small waters and snacks in and keep a spare nappy's and clothes in the car (unless its a bigger trip out and i haven't got easy access to the car easily- then i take my larger bag).

All my nappy bags have been backpacks and plain black ones. My original was my most expensive one- it was about $90 from a baby store - i can't remember the brand though. The larger fell apart pretty quickly (eg straps and clips broke and started to fray) but the smaller one is my one still going strong. All my other bags have been bought for under $30 on ebay- they have been my longer lasting ones. I think my most recent one is a Swallow (its in DH's car so can't check, sorry).

So i would suggest if you like the small/medium handbag style ones maybe get one for yourself that you can use for short trips and get a general larger one- thats not too 'girly' for longer trips and for DH and/or other babysitters to use.