Space Marines are the most played army in Warhammer 40k. They are the poster-boys (boys being the operative word if you read into their fluff) of the hobby and are the most pushed forward army. They are easy to paint, easy to pick up and fairly easy to play with relative success. Jack of all trades, master of none, Space Marines are a well balanced army that finds itself lacking when compared to the dedicated units of other factions. However, whilst a Genestealer will tear an Assault Marine apart in close combat, an Assault Marine can pick his engagements with the mobility of the jump pack and soften up a target on his way in with his pistol. Where a group of Dark Reapers will make Devastators look bad at shooting, Devastators can fend off Hormagaunts in close combat where Dark Reapers will be overrun quickly. Tactical Marines may be out-shot by Dire Avengers or Fire Warriors and out-done in melee by Hormagaunts, they can hold their own at both melee and shooting, where the aforementioned units can only do what they are meant to do, be it shooting or melee. Personally, I am not a huge fan of Space Marines, but I keep a small detachment of them just in case I want to play genetically engineered super-humans in giant armour with giant guns (both in the munitions sense and the muscles sense). I have a good amount of experience with Space Marines myself, having collected them when I was younger and having played against countless armies of them, I both have the keys to dismantle my enemy with them or dismantle them myself.
Whilst Space Marines are a forgiving army, they still require thought. Like the other factions, a Space Marine army needs to bring the right tools to the right situations. As your troops can do a bit of everything, you will need to leverage that against your opponent’s weaknesses. Your “dedicated” units, such as Assault Marines being a close combat unit, can’t go up against the dedicated units of other factions, the key to Space Marines is hitting your opponents weak points. Send Assault Marines after Fire Warriors, Dark Reapers or Guardsmen whilst your Tactical Marines and Devastator Marines hose down the Howling Banshees or Genestealers with copious amounts of bolter fire. Played correctly a Space Marine army is powerful on the table, though it also has the added bonus of making the journey from Neophyte to Veteran (basically, newcomer to Space Marines to a seasoned player) as painless as possible. I also enjoy playing against Space Marine players, as I have found less overpowered or cheesy lists fielded by Space Marine players. That is just my personal observation, not a total rule.