You ask a great question.
Sins online multiplayer is great fun. However, this game suffers from low player counts and does not have nearly the number of people online that it deserves. (Perhaps 98% of all purchasers only played it in single player.) The average player counts on a weeknight during North American prime time, say from 8 pm to 12 am U.S. EST, probably averages around 90 players split up between two servers and the three different versions of the game (the base game plus its two expansions). Also, many of the people who are online only play against AI with their friends. So, the practical effect of the low player counts is that you might have to wait for 1/2 an hour during prime time to find a 3v3, 4v4, or 5v5. What's really missing is the ability to play games on the types of maps you might want--such as on a multistar map or a custom map or with a mod; this game just doesn't have the player base to support that. However, people seem to enjoy the random single star maps so it's not that big of a deal. Obviously it will be harder to find games outside of prime time. Note also that most of the humans v. humans action (the real Sins game) occurs on the Diplomacy server amongst experienced pro players.
It's too bad that you missed the $4 sale of the base game that took place during the two weeks before Memorial Day; you could have purchased it and played it online before deciding whether or not to spend another $20 for the Entrenchment and Diplomacy expansions. Trinity might go on sale for $25 at some point but then you'd be gambling $25 on it.
Personally, knowing what I know about the player counts and the amount of humans v. humans online multiplayer activity, I wouldn't drop $25 on it today if I were brand new to the game and didn't know whether or not I would like it if my goal were to play it in online multiplayer against human opponents. (The purchase has two risks--that you won't like it, which is a risk you take with any game, and also that there won't be much online multiplayer activity.) That doesn't meant that you can't play in three or four 4v4 or 5v5 games in a single day or night--you often can if you are willing to wait patiently and you are diligent and you are monitoring the game activity at the right times. However, this game just involves waiting for games to fill and being patient, and with the player counts it has there's no guarantee that they'll be much online multiplayer activity three months from now.
I love the game and I have received much more entertainment value than the $62 I've spent over two years to purchase the base game and its expansions. It is still my top game today. I hope that you will purchase it and become a good addition to the online multiplayer community. However, in good faith, I really can't recommend it to someone who's primary interest is online multiplayer; I just don't think it's a very good buy at this point.
You should also be aware that this game has a high learning curve and that no matter how good you are against the computer AI, you will have a lot to learn to get up to speed against human competition. Some people have difficulty with that and you might get called a noob and type-yelled at in games. However, if you stick with it and diligently try to learn the humans v. humans game and improve you can learn to hold your own, especially if you are already an RTS veteran. There are still some new players out there, especially on the original vanilla Sins.
Player counts as of 10:35 pm U.S. EST on Wednesday June 16, 2010:
Sins/Entrenchment server: 36
Diplomacy server: 50 people, one 5v5 game is up but filled with 10 players haggling over who is on which team. (We often use a player draft system where two captains draft-pick the teams in order to get balanced teams.)
Total on both servers and all three versions of the game: 86
