Fun, detailed, absorbing, and extremely frustrating.
Most of the sidequests themselves are very bad. All of the optional worlds look the same. It's some bland terrain with one color for each world, and the side worlds consist of mineral nodes and random items. It's very, very repetitive. You land on the world, ride around in your hover car tank thingy, and find items / minerals and shoot robots. This game is probably the most frustrating game I have played in years. The auto save system is TOTAL garbage. I have had times where it literally took me back 2-3 hours. I was going through a story quest, and I saved at the beginning, and when I was near the end, I got stunned and killed. Most of the times the firefights are fair, and it's fairly easy to win, but sometimes you just get cheaped out and you die and there is absolutely nothing you can do about it. For example, I landed on a side world at one point, and I instantly died to some giant worm that crawled out of the ground. This would be OK, if the save system actually worked, but it's horrible in this era where gamers are used to having check points so they don't have to go to the pause screen and save manually every 10 minutes. The end result is you go through 15-20 fire fights, win rather easily, then you die to something stupid and have to do it all over again because you forgot to save.
The sheer amount of weapons and items you will find in this game is complete overkill. It's not unreasonable to find upwards of 50-75 weapons in one mission. The really annoying thing is how they categorize these weapons... for example, there are around 8-9 different "brands" of pistols, and they have a letter after them (V, or IV, or VII for example) that denotes their rank. But a Stinger I might be better than a Kessler V. Having to scroll through lists of weapons when you're looking at upgrades is just annoying. When you reach the item limit, having to go through and MANUALLY delete everything you don't want is very, very time consuming if there isn't a shop you can sell it to.
Mass Effect has good combat, a great story, and it's very immersive, but if you get frustrated easily, and you arn't great at remembering to save, I recommend you stay FAR, FAR away.