Advent is an interesting dingus. A brief review wouldn't really do it much justice, so here's a relatively lengthy one, starting with a bit of a bulleted list.
Sever Strong Points
> A fairly massive assortment of custom things, which are being added to on a weekly basis.
> A solid RP community which, though small, seems to be very accepting of new folks.
> General community friendliness.
> General GM friendliness.
> Relative freedom with character concepts. (See weak points for an explanation of the "relative" part)
> Relative lack of cliques. (These happen everywhere, hence the "relative" here)
> Relative server stability. (See weak points for an explanation)
Server Weak Points
> Creating anything other than a human currently requires GM permission.
> Having any abilities whatsoever outside of the basic skills of a character currently requires GM permission.
> Owning any magical/unusual objects of any sort currently requires GM permission.
> GMs are slow to respond to PMs, sometimes simply not replying at all.
> GMs are often unresponsive to requests to talk in general.
> Repeated Gravity Errors are a sporadic problem for many players.
> Massive event fights utilizing many, many enemies in realtime, rather than as Roleplayed fights. (This is a weak point due to the incredible lag it causes)
> Some issues with GMs not understanding written English entirely (This may have been just me: I use a lot of odd words)
Overall, the problems that I see here are very, very fixable. It boils down to needing GMs to be active in making their own system work (or to said system being changed to where they don't need to be more responsive).
The server requiring as many permissions as it does, for example, would not be an issue if the GMs in charge of those permissions checked their PMs often and replied to them promptly and helpfully. In terms of keeping certain destructive kinds of roleplay down, I can fully understand why the permissions are needed. A better choice would probably be to respond to abuses such as power gaming *individually*, but this would require more GM effort than simply needing permission for anything non-standard. In this case however, every race but human is considered non-standard. There is a fairly hefty list of things you can be, if you play on the server for some period of time beforehand and get a GM to give you the go-ahead. I have had remarkably little luck getting responses to my PMs, so I am unsure how hard it actually is to get that permission. To me this comes across as fairly controlling and restrictive, rather than helpful. A couple of bad roleplayers can't destroy a server. This is by far the biggest turn-off for me.
Community wise, Advent is fantastic. People talk ICly, respond ICly, and are generally active ICly very nearly all the time. I love this. I especially love the lack of cliques that players have going. There are going to be some anywhere, but on Advent I've yet to find any really solid ones. It bears saying again: I adore the RP community that I've seen on Advent. The only flaw here comes in the form of some of the GM characters. The storyline of the server revolves rather heavily around the actions and decisions made by non-player individuals, which leaves me feeling more than a little marginalized. No, I don't expect to be important after a month or two. It would be nice not to feel totally expendable, though. Which is how it currently comes across. One character, played by the head GM, is actually immortal. He's also named after the head GM. This smacks heavily of a Mary-Sue to me, which is worrisome, but not terrifically important to how I feel about the server. The bottom line here is that a plot centered around the actions of player characters, not forced along by GM decisions, would make the server better.
Which way this server goes will probably depend a lot on how the GM team responds to criticism. If it's used as a springboard to make positive changes, Advent will be an amazing server. If it's ignored or called flaming... that response tends to speak for itself.
Tl;dr version: Advent has a lot of potential and a fun RP community. The GMs can be fairly inaccessible and unresponsive. Give it a try, see if you like it.
NB: I won't be quitting Advent anytime soon. This isn't a flame, or a hate note. It will probably be updated as my opinions change over time.