I'm not sure if it will go 18 hours with the GPS on or not, but I usually charge mine and download all my training info to the computer about once every 5 to 8 days, and the battery indicator usually says there's still 60-70% remaining after that time. In fact, I don't think I've ever seen mine go lower than 60%. I'm not sure how accurate that indicator is though, since I've never let mine run all the way down(you're not supposed to let these type of batteries run all the way down).
I would imagine that with the GPS turned on and logging training data the entire time, the battery should easily go more than 10 hours... maybe even 18 as advertised.
I also don't like to wear the heartbeat monitor. I'm told that it's the same as the one from the 625M, although I never had a 625M to compare it to, so I can't say for sure. The heartbeat monitor that came with mine is terribly inaccurate, to the point that it's useless to me. Some people have bought heartbeat straps from Polar and used them with their 625XT and had great success with the accuracy of their heartbeat monitoring. I've actually used my 625XT in conjunction with an old heartbeat monitor strap from some small, unknown brand and it worked pretty well with that one.
I've never used the virtual trainer, so I can't comment on that.
The data fields are configurable. You can pretty much pick whatever you want and have it show up in there. There are options to show up to 4 fields on the screen at once, or you can set it up to show fewer fields but with a larger display font. Personally, I always set mine up to show "Elapsed Time", "Average Pace", "Current Pace", and "Total Distance". You can also setup multiple screens, in case you want to see more fields, and you can toggle through them by pushing one of the buttons on the side of the watch in order to access other fields in case 4 is not enough.
The backlight on the 625XT is extremely bright... you should never have any issues being able to see it due to a lack of backlight brightness at night. In order to send the info to the PC, you do have to set the watch into a Connect-to-PC mode, but the "Connect to PC: YES/NO" prompt comes up on the watch automatically when you plug it into the PC as long as the watch is turned on when you connect it, or if you turn the watch on after it's already connected.
I have never even seen a 625M, but the cable on the 625XT has 2 parts to it on the end that plugs into the watch. One is a single metallic prong, about the width of a ball-point pen head (possibly a tad smaller), and the other is a larger circular plug with a few pins inside of it. You plug it into the watch by flipping back a rubber seal on the back of the watch to expose a data port, and then you line up the 2 connectors (the ones I mentioned above) with the 2 holes in the back of the watch, and push on the back of the connector so they both go into their appropriate locations on the watch. It's pretty easy to use in general.
Here are a few pics of the connector that should make it more obvious as to what I'm talking about(My camera sucks.... sorry for the low quality imagery, but it's enough to convey the point):