A bit late on the discussion, but here are my £0.02 ...
I have an HTC S-620 (T-mobile Dash in the US) which I purchased in November, running Windows Mobile for Smartphone. Before that, I had a Nokia E61 (Symbian OS), T-Mobile KJam (Pocket PC) and Palm 600 (Palm OS). The HTC S-620 is quite similar to the Samsung Blackjack in formfactor and Operating System, so I think my reply will be valid to you.
Out of all of the above, I think it is the perfect solution for me! And I explain:
1) It is first and foremost a cell-phone. Small and light, it fits into my pocket easily. It has great reception, good battery life, good voice quality. 'nough said.. After the KJam which was bulky, I was pleased to have this small smartphone to carry around as my primary device.
2) As I use Outlook as my software of choice for GTD, full synchronization abilities are vital to me. The S620 syncs fully with Outlook, including categories etc. I have it setup to synchronize through bluetooth at the touch of a button, so I don't even need to connect cables to my computer. Once again, very happy.
3) I use the Calendar version that came with the Smartphone. The only software I've installed is "Oxios ToDo list" as the "Tasks" software that comes bundled is not very convenient. Oxios allows you to filter by category, non-completed tasks, undated or dated tasks etc. Perfect for GTD purposes and quite inexpensive.
To be honest, I mostly use the Smartphone as an Action tool - when I am out and about I look at my NA list to see what I need to do (@calls or @out for example). I also consult my calendar, make ammendments to contact details etc. For capture purposes I usually carry some plain record cards, as I find writing much easier. I process these separately at the end of the day or during the Weekly Review.
Finally, even though it is capable of push-email, I don't have this service setup as data costs are very expensive here in the UK, so I just check email only when I get back home. Also, I try NOT to do much processing on my smartphone, much rather do this back at home on my 19" screen than on the smartphone screen ...
Hope this helps.