I've played Marines a lot. Here's some things based on my experience.
-Their biggest problem is they have nothing between Battleship and Light Cruiser. Sometimes the Battle Barge can get left behind.
-Always build for your mission, so for missions where speed is the key (i.e. Escalating Engagement) Strike Cruisers are boss. For missions where you need endurance or hitting power, Battle Barges. Unlike other fleets, Marines have NO RESTRICTIONS on taking smaller ships to get bigger, so you can go 2 Battle Barges and have them buddy up in an Exterminatus! mission.
-Escorts rock your socks. Especially Gladius. Get more. They are your best hope against Eldar.
-Squadrons of 2-3 Strike Cruisers can be absolutely devastating. My preferred strategy is to move one Strike Cruiser to one side of an enemy and launch Torpedoes and Thunderhawks. Another Strike Cruiser is on the other side, and declares a Boarding Action. First Strike Cruiser shoots Bombardment Cannons, causing a Blast Marker touching only the enemy (not the other Strike Cruiser though!). Torpedoes and Thunderhawks hopefully cause a BFI (if they were on special orders already, this is moot). So, as Space Marines, you perform a Boarding Action with +2 for Space Marines, +1 for enemy BM, +1 for enemy Special Orders. Your opponent will probably get a +1 to +2 depending on their size and turrets, but that's a contested die roll where you can be at +3 to your opponent in most situations. Plus, BFI can't save against Boarding Actions.
-Battle Barges are slow and can't turn well, but I've won games because I used a planet to get the Barge to turn extra. Deploy Barges near planets to get 135 degrees of turn in one movement. (45 to start, another 45 after 15cm, and then another 45 after movement is over)
-Space Marines have fantastic Boarding Torpedoes and Thunderhawks (assault boats). Launch every turn you can, and team up squadrons for big waves to smash opponents. Even the dreaded Necron Tombship can't do anything if all its guns are turned off for a turn.
-Ram. You are armour 6. Cruisers are fast and maneuverable, so get behind them or to the side and take any opportunity to ram sides or rear. Emperor help your opponent if they're close to your Battle Barge and you can manage to Ram and end the movement with the base overlapping so that you can board as well.
-Look out for lances. Brace in most situations where lances are firing.
-Strike Cruisers are fragile. If using the 2010 update (DO THIS) pay the extra 15 pts for a 2nd shield. Makes them a lot more viable.
-Don't waste points on Terminator Boarding Parties. Although Marines rule at Teleport Attack (because of +1 to hit-and-run) actually GETTING a Teleport Attack is HARD. (there are a lot of conditions that need to be met) Buy more escorts instead, or cheap re-rolls.
That's all I've got for now!
