I have both the IPAQ Voice Messenger (VM) and the IPAQ Data Messenger (DM). The VM is my first non-touchscreen device and there are some significant differences. The non-touchscreen version of windows mobile (wm standard) seems to be more stable (not so many resets), but it does not provide access to as many settings. Many applications that are designed to run on windows mobile touchscreen devices (wm professional) are not compatible with wm standard, and there is not as much software available that runs on a wm standard device (in particular, GPS software options is not so plentiful). You'll need to search for the smartphone or windows mobile standard software versions.
After using both devices for a while (I trade off between them), I consider the VM to be a "lite" version of the DM. Of the two, I like the VM the best. It is smaller, feels great in the hand, and, now that I've gotten used to not using a stylus with it, I have gotten pretty proficient using the keys to get around the menus. Nonetheless, If you send a lot of email, text messages, or use something like twitter, you probably won't like having to use the keyboard to do this on the VM (the biggest drawback for me). The VM seems faster than the DM and the battery life of the VM is better than the DM.
The DM is a pretty nice device too, but the touchscreen has a different touch to it when I compare it to my other IPAQs (I have 7 different IPAQs). the screen is harder and I have to tap with a heavier hand. This is not a big deal and if I didn't still use my older IPAQs I'd probably not even notice the difference after a while. The battery life of the DM is not great, but I can get a full day's worth of use out of both the VM or the DM without having to recharge during the day. I don't have a problem with the keyboard at all. Every keyboard has a different touch, but once you get used to it it's really not a big deal.
Both devices have GPS (a BIG energy drain on all windows mobile devices), WIFI, and Bluetooth. I often use a bluetooth headset with them and don't notice much battery drainage when I am connected to the bluetooth headset, but, similar to GPS, wifi is a pretty big energy drain. So if you are heavy user of GPS or WIFI, you really need to use an adapter in your car and plan to recharge the device during the day. Another big drain of the newer 3G phones is 3G. The phone is always active if cellular wireless is turned on and 3G consumes more battery power than GPRS/Edge. 3G coverage can be spotty at times, and that can cause the wireless radio to consume alot of battery energy switching back and forth between 3G, GPRS, and Edge. If you aren't a heavy user of data services on the IPAQs, there is no reason to even use 3G and you can turn it off and use GPRS/Edge - that will save battery life.
I spend a lot of time on HP's IPAQ forum (maybe you've seen me there
posting as Prof Julie). There are a couple of threads on the forum that are really negative about these devices, but I can honestly say that I do not have a significant problem with either device. I don't know the history of the folks who are complaining about the devices, but I can tell you that you will need to reset your expectations in regards to speed and battery life. Switching over from a rx3715 to a windows mobile cellphone, you will find that the battery life is not nearly so great as it was on your older pocket pc. I still use an hx2415 and performance is about the same (although I don't feel as RAM constrained on the newer devices and don't usually have to perform soft resets on them).
With windows mobile cellphones, the device is always on standby mode if you have the Phone radio turned on. When you turn the device off, it will wakeup whenever someone calls or when a text message is received. You should also plan to subscribe to your cellular operator's data plan. There are some functions that will cause the device to automatically connect to the data plan (you can do some things to stop this from happening, but to be on the safe side, you should evaluate how you plan to use the device and subscribe to an appropriate data plan).
My first windows mobile phone was an IPAQ hw6925 (wm professional). When I first got the device, I also signed up for a small data plan - I am most often close to wifi and thought I wouldn't use a data plan. After a while I upgraded to an unlimited data plan (even though I am pretty close to wifi) because I found it extremely convenient to connect via my data plan. Using Google Maps while in a moving car makes a data plan a requirement.
If you decide to get the Data Messenger, you will be moving from a windows mobile 2003se device (the ipaq rx3715) to a windows mobile 6.1 device. Some of the software you used on the rx3715 will probably work on the DM, but you may need to upgrade some of the software to a WM5.0 or WM6.1 version (there were some significant changes that occurred in the OS when the OS changed from windows mobile 2003se to WM5.0 - the changes in the OS from WM5.0 to WM6.1 are mostly incremental and so most WM5.0 software will work on a WM6.1 device). If need be, you might be able to upgrade your existing winmo software free or for a nominal fee. Check the developer websites of your respective software to find out what your options are.