It has substance, but not much flash.
Star Ruler, like SE5, is extremely customizable for those willing to take the time and effort to fine-tune their game files to their own personal taste. Like SE5, Star Ruler allows the user to modify pretty much everything in the standard data files, as well as create entire new mods, race portraits, shipsets, and even particle effects. These can then be shared freely with other users.
GalCiv2 is perhaps the best 'sandbox' 4X game ever made, and like the original Civilization games, it really gives you the sense of governing an actual society in space. GalCiv2 is not quite so easy to customize, but the community has existed for a long time and there are plenty of maps, mods, races and ships available to download.
Star Ruler seems to be trying to capture the customizability of SE5 as well as the 'galaxy-spanning society' feel of GalCiv2, but doesn't quite succeed. Star Ruler possesses an innate lack of frills that borders on the boring; the game just doesn't have much of a personality or flavor. While it qualifies as a 'sandbox', it has only some of the map variation of SE5 and little to none of the sheer variety found in nearly all areas of GalCiv2.
There are no 'pre-set' races, as there are in SE5 or GalCiv 2; the race setup menu consists of a few bland drop-down menus for defining opponents' names, banner design, color, AI behavior, and difficulty setting. Sure, you can play around with their racial abilities or drop in some text to provide background information, but with only a few behavioral options, portraits, and shipstyles, one opponent plays largely the same as another. The maps are randomly generated, and there are some good options for manipulating the size and shape of the galaxy, as well as the ability to enable AI remnants and pirates.
Diplomacy is equally bland and unengaging. The AI players have no personality beyond their portrait, and everyone seems to speak the same language. Once you encounter another 'race', interaction consists solely of your opponents periodically spamming you with lopsided trade agreements that are usually beyond your economy's capability to maintain. This is particularly annoying in that early on, there is usually a clause stating that they will declare war on you if you fail to uphold the terms, and constant refusal of their overtures will lead to war anyway. There is no gauge or meter of your relationship with another empire, leaving you with no real way of knowing what to expect.
The ship design system, while not nearly as dynamic as GalCiv2's, is probably the game's strongest point. Once you get used to its peculiarities, it's actually pretty fun to use, if a bit time-consuming. It's particularly robust in the level of control it gives you over each ship design; you can set a ship's automatic commands right from the start, depending on its intended function, as well as designate how often to have your ships auto-upgraded as technology advances.
The difficulty curve is...interesting. By no means is Star Ruler a difficult game to learn. For anyone who has played a 4X game before, the interface and mechanics are fairly straightforward. Like so many other games in the genre, however, the gameplay essentially boils down to a race between you and the AI players, one which they are more suited to win. Real-time 4X games (such as Sins of A Solar Empire and Imperium Galactica II) have been done successfully before, but that success depends on having the proper interface for it - and Star Ruler's interface is just a little too clunky to make it easy to handle.
Even at the lowest difficulty settings (and even if you have the presence of mind to pause the game while rummaging through build queues) the AI still has the advantage in how fast it can propagate itself through a good two-thirds of the map. This is usually how it goes in most 4X games, but in Star Ruler it seems particularly hectic. In a large map with only a few players, the computer players can easily outmass you in territory, technology, resources, and warships in a relatively short period of time. (A mod to either regenerate ore or make it limitless would help as well - older colonies with heavily upgraded mining facilities can run out quickly, leaving you with a huge metal deficit and making a game unplayable.)
This is really not a micro-manager-friendly game, if you're the kind of person (like me) who likes to have each colony's facilities neatly parceled out and organized. When the AI expands to a new system every few minutes or so (usually sending entire colony convoys to settle four or five worlds at a time), the only way to really keep up is to do the same, focusing on mad-rush mass-colonizing and letting the AI governors develop your worlds for you.
As a game, Star Ruler is pretty much just plain vanilla. I would go so far as to say that it's probably best to play the game on anything other than the default settings; this is the sort of game where tweaking and add-ons are necessary to make it enjoyable for the individual player. If you're a modder, then it's definitely a good investment of money and time; with enough effort and a little help from the community, Star Ruler has the potential to be the game that each player wants it to be.