I have been play testing ghostorb for a couple of months now with members of my gaming group here is what I noticed. Mind you I was only testing operability. I am still working on mapping and avatars.
1). The wife is not too angry with me, since she does not have to see my D&D friends.
2). Ghostorb requires a lot of prep time, mapping etc which we as a group are working on. No problem for the computer savy members of the group but our DM is a total computer idiot.
That is to say he can't type and cannot follow computer instructions, no other real computer skills necessary!
3). The client is very good, very similar to the Heroforge spreadsheet, but without the macros.
4). We found it to be very effective in playablity so far.
Now for Some drawbacks.
1). It is not that easy to tell the DM where you want your character. However as far as I know they are working on improving that type of thing. Right now the DM controls the entire map and characters so in the heat of battle much communication will be needed to tell the DM where you want to be. If you make sure your map has a grid it can be easier.
2). I have not figured out how to upload NPCs yet. This will be a problem down the road. I am assuming it is in the works because they have said so.
3). All graphics must be ".png" type and a certain size. That in itself is not a big deal I only mention it because of the possible investment of graphing SW. I am currently experimenting with Autoforge which is free, others have used Campaign Cartograher, and Dundjinni as well as Paint shop. Which leads me to...
4). Eventually you will need to make a graphic for all the monsters, and NPCs. On a side note I would accept help from anyone out there on this one. Again not a big deal just very labor intensive.
5). This one is not too obvious but you make sure that all your gaming group has the same resolution on their monitors. If not everyone will see a different amount of your map which will lead to more time in communications than is already needed.
6). The DM will need a hard copy of the map with him because there is no way to store a surprise for the players without them seeing it ahead of time. So for traps and ambushes, it must be worked out on the Master hard copy prior to playtime so the DM can spring it on them when the players move into position.
7). You are also supposed to be able to upload adventures but I have not figured that out either.
With all that said to use Ghostorb currently you must have worked out a lot with your players before hand. Communication is even more of a priority than in person. The DM has even more work to do than before but nothing that cannot be improved or worked around. I like using Ghostorb and look forward to doing a full blown adventure on it not just these test games. I have never used the other online programs nor have I heard of them. I would like to compare them though.
I hope that helps you.