Here is a list of some SoD features from a post I made on the EZ forums a while back:
As for SoF/SoD AAs; there is no ETA on when those will be added in. Not even all of the Titanium ones are added and/or functional yet. AAs from the newer clients definitely could be added at any time, but I wouldn't expect them until the Titanium ones are all done, which will probably be quite a while still.
Getting the SoD client for AAs isn't really the reason to get it right now. The main reason is to allow you to experience any new content on any server that might use SoD features and content. Another reason is for getting some of the new built in features and also better hardware support and performance such as the new Multi-Core CPU Support and others. There are quite a few nice features from the SoF and SoD clients that didn't exist in Titanium. Here is a small list of some nice features off the top of my head:
1. The Group Window now shows Mana and Endurance bars for each of your party members, so you always know how much mana your Cleric has and what-not.
2. The bank has 8 extra slots for a total of 24 instead of the normal 16 from Titanium (tons of extra space).
3. The in game maps now have an option to show where your party members are if they are in the same zone as you.
4. The Loot All option now works when looting corpses.
5. New Power Source Inventory Slot available.
6. Heroic Stats and Resists as well as a few other new item stats that didn't exist in Titanium.
7. Dozens of new zones, new NPC races, playable Drakkin race, hundreds of new weapon models.
8. Much better lighting system. Once you see it, you will never want to be in the dark of Titanium again!
9. Not restricted to a max spell ID of 9999 like Titanium is, which means that reaching higher levels with full spells is possible.
10. Attack, AC, and Skills show correctly above level 70 (they show as bugged in Titanium).
11. Normally, zoning is noticeably faster on SoF and SoD.
12. SoD is more stable than SoF and even Titanium.
13. Spellbooks are 60 pages instead of the normal 50 in Titanium, which gives 80 more spell slots.
14. SoD has better multi-boxing support than SoF and probably even better than Titanium, at least for Multi-Core CPU systems.
15. The SoD client has updated textures for many old world zones to give them a slightly better look as well.
16. SoD clients get about 1k more HP/Mana/Endurance at level 70 if the server you are on enables the rule to match those stats to the formulas that the client uses for them (which changed since Titanium and SoF).
17. SoD has built-in multi-CPU support that makes it perform better than any previous client on multi-core CPU systems without the need for WinEQ or other multi-CPU core fixes.
18. SoD has an option to be able to copy the UI window settings from 1 character to another.
19. SoD is compatible with many of the current EQLive interface UIs, so you can have a wider selection to chose from for custom UIs.
20. SoD has frame rate limiting settings for background EQ windows to help make multi-boxing performance considerably better. It also generally uses less CPU than Titanium.
There are plenty more, but those are just a few reasons.
Keep in mind that some of the things listed there depend on the server whether they will take advantage of them or not. You aren't going to get access to a ton of new fully populated and finished zones, as they have to be created from scratch. Depending on the server, you may or may not see some of the things in this list, but that is up to the devs/admins to decide what they want to take advantage of.
Many of these features are not utilized on THF as far as I know, but some of them will be. The issue with AAs not being purchase-able on THF is due to the server running an older revision of the source before that was corrected, I think. Once THF updates past that fix, they should work fine. Really, SoD is better than Titanium in every way other than maybe in use of RAM, since the client does use more RAM than Titanium, which can affect players that box a lot of characters and push their PC's limits with Titanium already.
As for how many players run SoD, the numbers continue to grow. I think it depends on what content is available on the particular server on how many of each server's player run the newer clients. In the current revisions of the source, an admin can use the #cvs command which will show how many of each client is currently connected. Generally on my server and on PEQ, we see a pretty high ratio of SoD vs Titanium players.
I would not recommend using SoF though, since it has stability issues. The SoD client does not have those issues and is actually more stable than even Titanium is.
The only downside to SoD atm is that you can no longer get the version of the SoD client that EQEmu supports legally. Meaning the SoD that is on Steam atm is not the same version that we updated EQEmu to work with. So, there is no longer a way to buy the client, and the only ways to get it would be illegal, so we don't have an official way to attain the client anymore. The current SoD "Starter Pack" on Steam is now actually a fully patched EQLive client that includes Underfoot as well. While that new client is partially supported, it is not nearly as bug-free and complete as the SoD client that we do support is. So, I wouldn't recommend getting the version off of Steam right now either.