I had great hopes for Star Trek Online. But when I played the beta I was disappointed and didn't like the game all that much. Thus I decided not to buy Star Trek Online on release.
You might be surprised that I start this review with the conclusion, but actually most reviewers form an opinion first, and then write a review to fit that opinion. This is my very personal impression of Star Trek Online as experienced in the late closed beta, with no claim to either completeness or impartiality. I played about 12 hours, distributed over three days, which was enough to get a good general impression of the game, but certainly not all the details, thus this is just a "beta review".
Star Trek Online is based on two different engines, one for the space part, in which you control a ship not unlike the USS Enterprise, and one for the ground part, in which you control your avatar, and quite often a crew of NPC as well. Now you might remember reviews of Pirates of the Burning Sea telling you how nice ship combat was in that game, and how badly done the avatar part was. Star Trek Online unfortunately has the same problem. The space part and ship combat are nicely done, and different from classical MMORPG combat. But the avatars aren't pretty to start with, the animations especially in combat are wooden, and ground combat isn't much fun. One gets a definitive impression as if Cryptic Studios started with the design of the space part, did the avatar part much later, and ran out of time before they could make that part as good as the space part.
The world of Star Trek Online consists of various sectors of galaxy. In each sector you can see players flying around, stationary star systems, and mobile encounters. Thus gameplay consists of flying to one of these star systems or encounters, and entering its instance. Everything in Star Trek Online is instanced, even the galaxy sectors or the space stations exist in several parallel incarnations with a maximum number of players per instance. When you enter a star system or encounter instance, a scripted mission starts consisting of some sequence of space and ground tasks, which can be solo, group, or public quest. This is completely linear, you have to follow the script or the mission doesn't advance. For example you enter a system, get hailed by a freighter in distress, do space combat against some pirate ships attacking the freighter, beam down to the freighter to do some ground combat against the pirates in the ship, beam back up to the ship to fight off some more pirate ships, and mission done, warp back to the sector. If you enter the same sector again, the exactly same script starts, and you can do the mission again, over and over if you want. But you can also either hail Starfleet command, or visit them in the Earth space station, and get quests there. There are two sorts of Starfleet command quests I found, one a bit more elaborate versions of scripted missions, the other "visit X star systems in this sector and do the local missions there".
There are different types of rewards, two sorts of xp, two sorts of currency, two sorts of gear, plus you can get crew members as a reward. Experience points can be either for you, or for your crew. Your xp gain you levels, and every 10 levels a new rank, plus you can buy skills with them. The crew xp are a pool shared between all crew members, and it is you who decides the skills of which crew member you want to raise with them. Currencies are either energy credits to buy various gear with, or Starfleet merits to buy crew members and train them. Gear can be avatar gear to equip yourself or your crew members with, or various weapons, shields, consoles and devices to equip your ship with. Thus there is a lot of stuff to collect. You can also gather resources like alien artefacts from "anomalies" appearing in space systems and on the ground, which are used in "research" to upgrade existing gear.
Star Trek Online is not a bad game. It is somewhat unfinished in its current state, and still has some bugs, but that unfortunately appears to be the industry standard for MMORPGs on release. The space part with its ship combat is nice enough, and the general gameplay of doing missions, exploring systems, doing the two sorts of combat, and cashing in the rewards works reasonably well. However with everything being instanced, and the missions being heavily scripted, Star Trek Online feels very much like playing on rails. While theoretically you have the freedom to visit whatever star system you want, practically you'll only want to visit each system once, and that preferably as part of some patrol mission for greater reward. Thus you end up with a very linear checklist of things to do: Accept patrol mission, visit systems A, B, C, and D for that patrol, do the predefined script in each of the systems, get the reward, spend the reward on making yourself, your crew, or you ship better, and start over. This gets boring, repetitive, and "grindy" pretty fast.
The biggest disappointment for Star Trek fans is that STO reduces the drama, dialogue and interaction of characters of the series to combat. Star Trek Online is a simple combat game; the only non-combat interaction possible in the game is getting close to an object or person and pressing "F". In most cases there are no dialogue options beyond the choice to either accept or cancel quests; there are very few quests in which you have to talk with NPCs and do a little multiple-choice dialogue to solve the quest. There is nothing to do on the bridge (as of the beta), nobody to talk to, and if you want to sit in the captain's chair you need to stand on it and use a /sit emote. And instead of boldly going where no man has gone before, we will quickly see some website listing all the quests on all the systems.
So while Star Trek Online is certainly playable, there is a chance that you might be disappointed by its linearity and shortcomings. I am posting this review at the time where the NDA drops and the open beta starts. So I can only recommend you play the open beta yourself, and form your own opinion of Star Trek Online.