I think it's pretty decent as far as strategy game AI goes.  It's better in the regular game than the Entrenchment expansion since it doesn't always make good decisions about when and when not to attack starbases and how, but that's not a killer since the regular game itself is very worthwhile alone.  What are you looking for in a computer AI opponent?

You can create very difficult games for yourself by upping the difficulty level and/or allying large amounts of opponents (up to 9) against you.  Of course, that's kind of artificial it won't be the same as starting off on a level playing field.  AI will never be able to rival the kind of challenge you'd find from playing against human opponents in online multiplayer--humans won't do silly things and will make much better decisions and might actually have a game plan.

Consequently, if don't want to ever face off against other people, you can play it online in humans v. AI games called "comp stomps" which can be set up to be challenging--3 people v. 7 Unfair allied AI.  The game might be more fun with human allies.  Another nice thing about this game is that there are some nice mods out there which really only work for single player, such as the Seven Deadly Sins mod.  I'd definitely look into that one.

You could try AI set to Unfair level and set to "Aggressor".

I don't mean to be impolite, but is there a reason you're not interested in considering the online multiplayer game?

To be honest, I'm not sure "cheating" is possible in sins (I could be wrong). As the game has mod support built in, the game always checks the game files and gives a "checksum" that is different for each mod. Players that have different checksums can't play each other, so I would think modding the game to your advantage would do no good.

AI is fairly good. its not the best ive seen, but Sins is very complicated from an AI point of view.

however, the AI does scale well with difficulty and behavioural type. for instance, an unfair fortifier will build a fortified world that will take your about 40 minutes to crack, whereas a hard aggressor will be throwning fleets of LF and LRF when you are still playing with 1 cap, 5 LF and have about 4 planets colonised... meaning you really have to jump to to defend yourself.

economists are good for large fleets, while researchers are good for advanced ships, structures etc.

basically, if you play 1v1 with an unfair AI, whatever the behaviour, on a fair and balanced map. you will most likely win (once you've gotten some experience up, of course. an Unfair AI will destroy a first time player) unless you muck around and dont pay attention, for instance. this will happen in just about any game you play vs AI.

however, play against a few (up to 9) unfair ai, all allied against yourself, and THEN you will have a fight on your hands. unfortunately, its still a numbers thing, however, you have strategy up your sleeve, so you can choose where to fight (for instance, fighting in a plasma storm will disable strike craft and therefore, if you have built your fleet accordingly, give you an advantage, or you can set up some TEC starbases along some dead asteroids near your central planets and use their big red button ability to thin the enemy fleets befor eengaging them) there are all sorts of things.

all in all, i think the AI isnt all that bad, but of course, it isnt as smart as a human. however, if you want to win every time after a bit of a fighht, its a good bet.

a good thing to note too, is that you can change the difficulty and behaviour of AI after a game has been started. you just save the game an exit, load it up agin and in the pre-game lobby, just change the settings of which AI to what and play. you can also start playing as one of the AI players. so the AI takes your planets and ships and you take his and you can make what changes you want personally, thne go back to your AI.

for me it's fairly good although it's not really the very best i've seen.

Compared to other games, the AI in Sins is very good.  Compared to a reasonably experienced human player it's a impotent worm. 

The specific weaknesses of the AI is that it doesn't expand its empire very quickly, picks poor capital ships and capital ship abilities, chooses the wrong unit types for its situation, and doesn't manage its economy very well.  Perhaps ironically it is situationally too timid (doesn't attack when it needs to or has an opportunity) or too aggressive (attacks in situations where it's clearly outgunned or running into an obvious death-trap).  However, its most glaring weakness once you start using advanced strategies is that it fails miserably when trying to fight a two-front war.  In the expansion pack its ability to utilize and counter starbases is very poor as well.

That aside, it does know how to build a reasonable economy and empire and use its fleet as a coherent force.  Though it's not very good at deciding when to do so, it also retreats.  This is more than you can say for most RTS games.  Even very recently many RTS AI's just send endless waves of units to meander towards your base, which are easily dispatched by a human player's army.

I don't get people who say it's not the best AI they've seen.  I've played countless RTS and Sins is the best AI I have encountered.  When you get experienced at the game, a couple of games, maybe more, the AI will start to need to stack odds in his favour to make a challenge such as stacking teams or higher difficulty levels that cheat.  Par for the course with RTS though.

As for cheating online I'm not a multiplayer player but a frequent forum user and I've never once seen someone complain of cheating.