I currently have the Blackjack II and I love it!
It does everything a basic smart phone does and then some. It does the typical things like browse the internet or download applications onto your phone (gps navigator, weather, or even an application that will tell you what local stores are having special deals or coupons) and since its a windows mobile it will sync with your microsoft office files such as word, powerpoint, excel and will even read pdf files (this is super useful for me since I'm a student and this allows me to edit my microsoft word files on the go without needing a computer). You can sync your e-mail accounts on the phone. The phone allows you to set your Mp3 songs as ringtones as well. The camera is also of good quality and even has some photoshop type function to adjust brightness or crop the picture even. The camera also doubles as a camcorder as well.You can even log on to AOL IM, Windows messenger, or Yahoo messenger. The games include solitaire and bubble breaker as well as a demo which you can buy the full version of a brain challenge game similar to Brain Age on the Nintendo DS. The battery life is pretty long...on a full charge I was able to get a full 2 days out of it and I talk a lot as well as using the applications on it. There are just a tons of features that I haven't even gotten around to playing with yet but so far I love everything I've found.
Before my Blackjack II, I also had the Sidekick 3 which I liked but it did not have a lot of capabilities especially for the price tag and being a full QWERTY phone. It was very basic: internet, AIM, text, calendar (but no alarm, you have to purchase the alarm), music player, games, camera, etc. I just don't recommend a Sidekick even the new ones because nothing has been changed on the new ones except that it has a new exterior look. Well, a Sidekick can be good if you're just looking for a basic phone with a full QWERTY keyboard and don't need the extra applications.
The iPhone seems like a good option as well. My boyfriend has one but I think it's just preference. I personally don't like the touchscreen and find it a pain to use (it's easier for me to text/type with actual buttons than touching the buttons on the screen). Other than that, the iPhone has a lot of functions as well - GPS, weather, youtube, internet, stocks. Recently, the screen on my bf's phone also froze on him and I've heard that it's common but maybe it's been fixed with the new versions. He sent it in and got it back fixed pretty quickly though. Before the iPhone, my boyfriend had the Blackberry curve as well and he didn't seem to like it all that much - I'm not sure why except he sold it and got the iPhone instead.
I think it just depends on what you need your phone for. If you're not going to use all of the features, there's no point in spending a tons of money on a smart phone with applications you'll never use. When I was younger, I loved my Samsung slider phone SGH-T09 because it performed all the basics and even had an Mp3 ringtone capability but now that I'm on the go and busy all the time, I find that having a Blackjack II is useful since I'm not always at a computer.
Sorry it's so long. Hope it helps!