I'm thinking of switching to an Apps account but I'm worried about the complexity of the transition for all my data.. Do you have any advice for that? Also do you miss anything with an Apps account compared with a regular one?
(1) you can import your email from any account (that supports POP3) in a new GMail account. Just go to Settings -> Accounts in your new GMail account and configure it (note that the old GMail account also has to have POP3 activated). Actually from a new GMail account you can also send email using an older email account by SMTP. So basically, with a new GMail account, you can keep using an old email account from within the same interface, if that's what you want (heck, you could use a Fastmail.fm account from within a GMail interface, if you don't mind the slight latency).
Make sure that you label these emails with a special label, such that you can filter them later more easily.
(2) on your old email account, you can set a "Vacation Responder" that informs people of your new email address automatically.
So yeah, if you like GMail, then don't wait and do migrate to your own Google Apps domain. You won't get ads anymore in GMail, you'll have the option to migrate without losing your email address and you'll be treated like a customer (in my experience that doesn't necessarily match other people, their support has been very responsive).
And $5/month is not very cheap, but on the other hand it's the price for 1-2 coffees and I value my email. I do prefer the "Flexible" plan and not the "Annual" plan, because the later imposes a termination tax. So do it, don't wait.
It's the same as when changing the phone number, people will just keep calling you on your old number that doesn't exist anymore and that complain to you that you didn't call back.
As a side note, the lack of support for legacy emails in Gmail and Contacts is really bothering me. I would like to be able to mark someone's mail as inactive, so that i can see these mails when I look for old conversations with the person, but not to show up in autocomplete when I try to actually write an email.
I exported some email filters and all of my calendar events. It's also possible to export/import your email as well. For your google drive you can just share everything or save it on your pc and put it in the new Google drive folder.
The things I miss are really small. You might have a bit later or no access to beta's and new features are not turned on automatically (recently this was the case for webhistory, you had to push a switch in your Apps control panel).
Furthermore you should probably try using a catchall. I like it a lot, you can filter on specific sent to emails from different services. And you always know who sells your data. I use the format company-type@domain.com. This way you con filter on sent to emails that contain shop for all your online orders.