The interface of the D840 is the same as the one seen in recent Samsung phones. Samsung doesn't seem to have given the phones anything to distinguish one from the other. Navigation is simple though – everything you need can be accessed from the menu.
I would like touch upon one thing that all slider phones seem to miss out on: the unlocking mechanism. While it is simple enough to slide the phone open and shut for unlocking and locking the phone, you also have the option to unlock the phone without having to slide the phone open. But then to unlock it again you will need to slide it open and then shut. Rather silly if you ask me!
Messaging can be a pain due to the non-responsive keypad. Also the T9 is pretty slow, making the experience that much less enjoyable. One of the coolest features of the phone is the fact that it remembers the last used feature in the menu, and highlights it when you access the menu next.
Now there is something peculiar about the phone that really doesn't work for me. Whenever someone calls, you can either accept the call or reject it. Most phones include a 'silencing' feature, where you can simply stop the ringtone, while not hanging up on the person. This way you can show that you were busy. But if this phone rings while you are in a meeting, you will either need to take the call or reject it.
The phone can take up to 1000 callbook entries. In messaging, the phone has an option called Vivid Messaging. Selecting this allows you to view certain words animatedly, like 'love you'! I have never really enjoyed browsing on a Samsung phone, and this is no different. The phone supports EDGE with GPRS.
As in the LG Shine, this phone also has the Mobile Tracker feature. You need provide an alternative mobile number (presumably your spouse's) and name. Every time someone (including you) replaces the SIM in the phone, a message will be sent to the alternative number with the location of the user and of course the number of the new SIM card.
The D840 boasts a 2MP camera, which is quite decent. A flash makes it possible to click pictures at night. The pictures, although quite grainy, don't look overexposed as is common with most camera phones. A lot of effects like Sepia, Negative, Moonlight etc are offered. The music feature almost seems like an afterthought though. The phone supports A2DP – I tested it and wasn't impressed.
Lovely phone except the battery. have been using it for nearly 7 months now, was one of the first purchasers. Bought 4 more within the family and am in love with it,. does not hang, does what you ask it to do, unlike Nokia and sony phones.. Sadly the charger cover is a bad bad design .D-840 does have a silencing' feature, just press the volume key down for a second when the phone is ringing. 
