Okay, this won't be the greatest review anybody's ever read, but since GS3 doesn't have one yet, I figure I should post one to give everyone an idea of what the game is like.
GemStone III is a large (both in terms of size, and player population), high quality, commercial MUD about 14 years in age. Few games I've played match it in quality and polish, or overall depth. Simutronics knows what they're doing when it comes to online gaming, and it shows when you play GS3.
The first question a person might ask concerning GS3 is, "Why should I pay $12.95 or more every month to play a MUD, when there are a few very high quality free MUDs out there?"
It's a valid question, and many will find they won't be able to justify the expense. However, there are many ways to answer that question.
As I previously mentioned, the game is quite polished. The combat system has a great deal of depth, as do the supporting systems (spellcasting, treasure system, health/injuries, societies and guilds). While not perfectly balanced (such a thing in a game with as much depth as GS3 probably isn't possible), the staff does go to great pains to make the game challenging and enjoyable to all.
The game follows a tried-and-true level advancement system. You earn experience (mostly through combat, though experience can be earned through other means) and when you have enough, you enter the Character Manager to level up. You have a pool of training points that you can spend on improving various skills, such as Edged Weapons, Blunt Weapons, Twohanded Weapons, Armor Use, Shield Use, Two-Weapon Combat, Climbing, Swimming, Hiding/Stalking, Lockpicking, Disarming, Perception, Mana Sharing, Magic Item Use, Scroll Reading and Spell Research. Costs vary depending on your class, as does spell availability through the Spell Research skill. Though not the most original or innovative system used by a game, it does a fairly good job of keeping things balanced, while still allowing a fairly wide range of training options to make your character viable and unique. It's suspected that soon a major revamp of the Character Manager system will be made to re-balance the game.
One of drawbacks that the staff has to contend with in GS3 is the fact that the game is so old. There is a lot of inflexible code in the system that often makes implementation of new ideas difficult, though the game staff is making a constant effort to root out old code and find ways to work around the limitations of the old.
Overall, the staff seems pretty successful. They are constantly implementing new features, which is readily apparent if you take a look at all the things they released over the summer last year (http://www.play.net/gs3/news/hotsummernights/hot_summer_nights_releases.asp).
The staff, by the way, is outstanding. They are professionals, and despite the game's large player population (you can expect anywhere between 300 and 1300 players to be online at any given time), there is a reasonable chance that you make interact with a staff controlled NPC in some manner or another, if you make the effort.
Because of the frequency of staff run events, there is always a feeling that something at least somewhat unique is occurring. The events run by the staff are many... sometimes a merchant will pop in to town, selling exotic wares, or performing some service... one popular service is "alterations", where a GM (in the guise of an NPC artisan) will customize an item in your inventory to your preferences. Other events of greater import occur, including invasions and great sagas with storylines that take place over the course of months or years, in which you try to play a part.
Role-playing is encouraged, though not required by the game staff. Blatently out of genre remarks are against the game policy, though there is a lot talk and behavior in the game that will make a die-hard role-player cringe. However, few games have a better support system in place for role-playing: the game's history is vast and deep, and a great deal of effort is made to keep the game culturally and historically consistent with itself. There are tons of social verbs, and a DEMEANOR system that will determine whether or not your character turns your nose up at someone who tries to kiss you, or accepts such gestures warmly. You can vary your character's tone of voice. Your character can stargaze or look at the moon. There is environmental and day/night messaging, with room descriptions consistent with the time of day. You can play roshambo or dice (if you have dice). There's a LOT of stuff to do besides killing stuff and climbing the level ladder, even if that is the thing that gets the most attention. Role-playing may be something you have to try to look for a bit in GS3, but there is a very active community of role-players that you can involve yourself with if you make the effort and keep an open mind.
As mentioned above, the game costs $12.95/month for a basic subscription. You can pay up to $32.95/month for a "premium" subscription, which has a lot of nice perks, though 99% of the game can be experienced and enjoyed with a basic account. There are also "ticketed" events, like Hunt for History or the Ebon Gates Festival (typically priced in the $40-80 range) and though one might balk at these prices, or the concept of paying to participate in an event in a game--the events themselves are in all honesty and without exaggerating unlike anything you would otherwise experience in a MUD.
I should touch on PvP as well. In GS3, you can more or less attack anyone you wish, though simply going around attacking and killing people for no reason will get you warned, or eventually locked out of the game. In general, Character vs. Character conflict, where the players are role-playing a conflict out with one another and AREN'T fighting because of a grudge between the players involved (or as a test of player skill) is fine and even encourged, while PvP that does not involve role-play is frowned upon. Unconsented PvP is against the rules.
Well, there's much more that could be said, but that's all I'll say here. This isn't the most eloquent or complete review, but it should do until Juspera re-submits hers. :)