The 8100 was a rather surprising phone. I really didn't want to like it because it goes against my tendancy to have utilitarian gadgets that do a few things well (so a normal sized swiss army knife knife is good, a 100+ function mega-multi-tool is overkill because you can barely hold it in your hand). The 8100 has so many bells and whistles that it comes perilously close to having too many functions, but it manages to incorporate them in such a way that I'm finding myself really enjoying what this phone can do.
First of all, the actual phone works well. It's easy to make calls on it by dialing, using speed-dial, and voice-dial. It has an easy to engage speakerphone that works well (or at least as well as a speakerphone ever does) and a convenient 1-button silent/vibrate mode that makes it easy to shut-it up when needed. I've read some complaints about the voice-dial system, but on the whole I think it works fine. It's different from other phones I've had, but it's easy enough to understand/use and it gives you enough flexibility to call any number in your phone book. When I bought the phone I also picked up a Jabbra 250 bluetooth headset and it interfaces with the phone (and the voice-dialing) nicely. I've had a couple of non-phone related issues with the headset though so I'm not sure if I'd recommend that particular model. However, a bluetooth headset of some sort is very nice with this phone and I can certainly see why so many people have jumped on the bluetooth bandwagon.
Beyond calling/dialing, the phone has a good sized phone book with space for multiple numbers (and email addresses) per entry (this comes in handy for other functions). You can customize ringers, txt message notifications, and picture IDs for all entries (all of which can be used for amusing effect with the right files ;). The external LCD screen displays call information, caller/picture ID, and message notifications (e.g. text messages, missed calls, voicemail, etc...). The external buttons beneath the screen can be used to acknowledge those notifications and you can read text messages without having to open the phone (a surprisingly nifty feature).
The text and picture messaging are solid and it's easy to send/receive message both inside and outside of the network. It's also easy to email photos and text messages to any standard email address (although if you're going to do this with any frequency adding the address to your phone book will make your life a lot easier). I use this feature for making picture posts through LJ and so far it's worked perfectly every time and it's easy/fast to write up a quick message and send it using the T9 word prediction software that's built into the phone. Photos come out nice and clear (example 1, example 2, example 3, example 4). I don't know that I'd actually print these out for framing and display, but IMO, they're pretty good as snapshots go and the 1.3 Megapixel camera is actually a tiny bit better than the one I had in my first digital camera just over 6 years ago.
The single feature that I think really makes this phone cool though is it's removable memory slot. It can hold a mini-SD card (I have a 1 Gb card for my phone) and it can be used to store all of your pictures, movies, mp3s, etc.. Unlike previous phones where you had to either connect the phone to your PC via wire (and load software) or email things to yourself, the 8100 lets you put all of your media files on a single card which can then be removed and accessed from your PC. You also need the card in order to use the phone's built-in mp3 player (and in some cases you may need to go to Verizon to have the software changed in order to enable this feature). Right now, I have about 30 mp3s, 30 music videos, 3 full length movies, and about 80 photos/pictures and I'm using less than 400 Mb of the card. I'm sure plenty of folks would disagree, but now that it's simple (and cheap) to move media files between the phone and a computer, a lot of features become much more valuable and useful. Curiously, it's one of the few phones that I've seen that uses mini-SD cards, most seem to use the smaller and harder to find Transflash cards (as of this writing I can easily find 1gb MSD cards and only 512 Mb TF cards).
While I won't say that this phone is perfect, it's certainly exceeded my expectations and I'm finding more and more ways to enjoy using it. From using it's mp3 player, to taking random amusing snapshots, to talking for hours with friends and loved ones, this phone pretty much lets me do everything I'd want to do with a phone and it's even made me reconsider what I'd want a phone for. I'd definitely recommend this phone to anyone looking to get something new.