Getting Started

I'm coming from yaaic

Not a whole lot has changed since Atomic forked from yaaic, however there are some things to note:

SSL support has changed: you are now prompted to confirm the SSL certificate. At least check the first and last few bytes.
There are color schemes other than the default one to choose from, including several which are more friendly towards AMOLED displays.
There is an option to hide the channel bar, opting instead for a more compact layout
Interface styles are more keyed towards the Android default style, which may be influenced by your OEM (e.g. Samsung, HTC) or your ROM (CyanogenMod, Slimkat, etc).

If you've used other android IRC clients, you should be fairly familiar with Atomic already. Things to check are:

Ident is the "Username" portion of an IRC name -- this is changing in 1.19 to be "username"
To truly exit Atomic, press back twice at the server list. You will be prompted if you press back once to press it twice to exit.
The default text encoding is UTF-8 -- If you have a server which requires plain ASCII for proper connections, you need to use the encoding US-ASCII
Atomic by default only reconnects automatically when you lose network connectivity entirely, not change between networks. This option can be changed in the settings.
By default, no sound is played when you are highlighted in a channel. This can be changed in settings > Highlight
For compactness, timestamps are not turned on by default. This can be changed in settings > interface