I was a Treo 650 user and recently switched to a Samsung Blackjack. I was initially unhappy with some of the missing applications, but have found out how to do pretty much everything I could previously do with the Treo and more. Most of this was learning from the blogs and web postings of others with more time than I have, but I've figured out a few things on my own.
First, Finisar uses Microsoft Exchange for their mail server, and, unfortunately, I was already using MS Exchange for the university. I couldn't find any listed way to support two Exchange servers posted anywhere on the internet, but was able to make it work. Here's how: Finisar also supports good mobile messaging (aka goodlink) www.good.com. By setting up the Finisar mail server via goodlink, and the university mail server via MS Exchange (outlook mail on the blackjack), I can receive mail and syncrhonize mail, calendars, contacts, etc, all over the air. This way, I can maintain a univeristy calendar (that my seceretary can see/edit) and a company calendar that is not visible to the unversity exchange server users.
I was missing the dialup networking ability of the Treo 650 over bluetooth and searched around until I found that the blackjack can do essentially the same thing, though it's not bluetooth DUN. Instead, you use the internet sharing capability under windows mobile. Goto Start->Applications->File Explorer, then select "UP" (left shift key), then Windows->Internet Sharing. From here, you can either select "USB" and tether to your PC over the USB cable. Once you connect the cable to the PC and select "Connect", a "windows mobile internet sharing device" will appear in your networks (under windows XP -- this probably won't help if you're not running a windows operating system), just right click and enable it. For bluetooth, first pair the device with the PC, then open internet sharing and select "bluetooth PAN" as the "PC Connection". Select "Connect" and then in your Bluetooth network places, enable the blackjack as the bluetooth network access point.
Some useful links that I've used to fix the memory problems with the
blackjack, namely to put the mail storage and IE cache on the memory
card and change the screens while dialing and during calls, so I don't
need to remember which keys correspond to which letters when prompted
to spell something using the keypad. These alterations begin with
modification of the windows registry on the phone, so caution is advised.
Samsung Blackjack Hacks
Low Memory Warnings on with Windows Mobile 5 Smartphone
Jeff Atwood's Samsung Blackjack Tips and Tricks