Are you required to 'sign in' to use it? If not, is there a difference in use ('tween signin or no)?
Signing in links your Chrome instance to your Google account. No, it's not required. The main difference is, you can synchronize bookmarks, histories, and optionally (with an additional login) password info across browsers on all platforms - Windows, MacOS, Android, even Linux. For example, I have Chrome on my desktop, netbook, laptop, and DVR. When I'm signed in, I can update a bookmark on any one, and it will appear on all the others. I can open it up on my Android phone, and have all the same bookmarks there as well.
You can also optionally synchronize plugins and extensions through your account: if I'm on my netbook and find a neat plugin for, say, displaying a photo's properties and EXIF metadata... I can install it on Chrome, and it will automatically download and install on Chrome on my other Windows machines as well (obviously there are OS compatibility issues with some plugins).
Sign-in also links your Google Cloud Print account, if you want it to, so Cloud Print-enabled printers are accessible to you from anywhere. I can read something on my phone, say, and then send it to my home printer via Cloud Print, so the hardcopy is there waiting for me when I get home.
When away from your device. Do you load Chrome or can you have it on a USB stick for that?
There is a "portable" (USB stick) version of Chrome available, although I don't generally use it. I'm more likely to just install Chrome when I need it.
Was there any features you lost converting to Chrome?
The ONLY things I miss that I had in Firefox, were a few specific plugins that have never been duplicated in any other browser (that I've seen). One is called "Down Them All", a fantastic tool for performing "web sucking", where you can bulk-download images, links, and other items from a website. Another is "BBcode", which adds BBcode formatting options to a popup menu: for example, in HTML you use the < b > tag to bold a block of text; in BBcode, you use [b]bolded text[/b]. The BBcode plugin lets you just highlight a block of text, right-click, and select the desired tags.
What is it that you find lacking chrome?
Other than those plugins... nothing.
Do you use the smartphone app?
I do not... because strangely, I find Chrome for Android's performance to be TERRIBLE on my phone. It might have something to do with it being an older phone (two-year-old HTC Desire HD), or the fact that I'm running a third-party Jellybean ROM... but Google's browser does not like my Google operating system .Chrome for Android has some quirks that annoy me, too, like ALWAYS reopening old tabs when you launch it from a new link, meaning unless you regularly manually close tabs, you can end up with a LOT of them, and that REALLY kills performance - as yet, I've not found a way to change this behavior.
On the whole, Chome is easily my preference, and like so many others, I only use IE when it's required by another app or hardware. In fact, I've found that with Axis cameras, it works MUCH better than IE - Chrome can use Axis' MJPEG push to view video with almost un-noticeable latency, while IE won't view MJPEG or H.264 video from them without installing a plugin of one kind or anohther.