I’ve been playing with iPhone OS 4.0 on my iPhone 3GS since it became available to developers after the press conference yesterday and so far impressions are good – it has broken the Worms app which won’t even launch any more 🙁 but everything else seems to be fine.
Multitasking pause/resume of apps seems to work quite well and is a welcome improvement, it will be interesting to see what happens once developers start to take advantage of the new APIs.
The folders/groups/stacks of apps is a great addition for those a little bit OCD who like to keep the apps on their phone organised. It’s already cut the number of screens on my phone from six to two (which could technically even fit on one!) which makes navigating between apps a lot easier.
There’s an interesting addition I’ve noticed where you can now set “complex” unlock passcodes which allows longer, alphanumeric strings to be used.