Assault Squad - Bad
These guys are almost okay. They break the core Marine rule (be capable of anything) unless you bring expensive pistols, like the unreliable grav, or the dangerous plasma. Blood Angels help make these guys passable, but that's only due to melta access (and being a troop!). Sure, they adhere to the first rule of assaults by accomplishing turn 2 charges, but that's really not enough to help make them more viable. This is largely because they're only better in close combat than a tactical squad, or anything worse in CC than a tactical squad, which is to say xenos troops. If you really need to punch those in the face, drive your rhinos faster or jump into drop pods, but don't waste time with the assault squad.
Land Speeder - Okay

Stormtalon - Okay
Oh boy. Here comes the big discussion on Marines and birds. Right, so, the best place to start is by considering which enemy flyers cause problems for us, and what things in the game can our flyers cause problems for. Which flyers cause real problems for us? The Heldrake. That's it. Yes, I know, the Vandetta is good, but the most it will do is drop guardsmen in your deployment zone after killing a marine (maybe two) or destroying a tank. Your rhinos are likely to be within its "minimum range", and your predators or dreadnoughts should be able to claim cover. Likewise, if your Space Marines in your deployment zone cannot handle a squad of enemy Guardsmen, you have deeper tactical issues than what this review is designed to address. So, the Heldrake, what do we do there? We don't consult the Stormtalon. We do not consult the Stormtalon. It's only a paltry few points cheaper than the Drake and it is not the drake. Yes, the volume of fire it can spit out, mated with Strafing Run is impressive (the lascannon is a trap, you want a volume of midrange shots out of this guy, because boo stupid assault cannon), but the fact of the matter is that the little bird dies to a single vector strike attempt from the Drake. What then do we do if the enemy brought Dragons? Ignore them. Kill the troops. Force the game into at worst a draw.
Why would you bring the Stormtalon? If your area is low on interceptor, you pay the skyhammer missile launcher tax, and not unlike a rifleman or speeder with assault cannon and heavy bolter, spit out fire which terrorizes xeno infantry, harasses light and medium tanks, and if nothing else, forces pinning checks. Is it the most efficient thing in the game at this task? No. Is it generally at least as good as a rifleman for the reasonable tax you pay? I'd argue that yes, it is. You really really don't want to take snap shots with this guy, but generally, they have a place. Just don't expect them to be a Vandetta or Heldrake. They are not, despite what their similar points costs suggest.
Bike Squad - Great
So, if you haven't heard, these guys are currently the bees knees of troops choices(personal views on the fire warrior not withstanding). For time and a half of what you pay for a tactical marine, you generate your own cover save, twin link your boltgun, gain relentless, move 100% faster, generate hammer of wrath hits when charging, and are 25% tougher. I can also bring two special weapons, regardless of how many models my squad contains (plasma and grav, eat your heart out). Sure, I can't get in a tank, but I have most of what the transport offers built-in: namely speed, power, and durability. What's not to love? Oh! There's the added perk that by buying a bike for your captain or chapter master (a meager tax, especially for the T5 it offers a model which can be your warlord), squads of at least 5 bikes are taken as troops. Then there's the option of bringing an attack bike, which can give your grav gun toting guys a relentless multi-melta, helping round out your unreliability against tanks. The attack bike is hit or miss for me, especially if your play area is overrun with Wave Serpents, but is really worth considering as part of a troop choice.
Attack Bike - Okay
Remember the land speeder? The attack bike is its little brother. Similar points costs. Similar weapons (with the biggest draw being a multi-melta). Similar durability (vulnerable to small arms but harder to destroy with a single lucky shot). Benefits? It can claim chapter tactics! White Scars and Iron Hands get the most out of these guys by far. Otherwise, it's all about personal preference and available points between these guys or the speeders.
Scout Bike Squad - Bad
So, you can save a few points and put scouts on bikes. Why is this so bad? They can't be troops, their grenade launchers aren't nearly as good as they used to be, they can't take real special weapons. Oh! And the other reason, besides the grenade launchers, you used to take these guys? Cluster mines? Now they tell your opponent which piece of terrain they've been used on, and they only cause it to become dangerous. If you want to take dudes on bikes, take the power armor guys, even if they're not troops. The few points you save aren't worth all the drawbacks of being a scout.
Summary
Well, this is awkward. The only unit here to be better than okay are bikes because they're a troop swap. Sure, they're mostly excellent in their own right (barring putting infantry targets on a potentially tank saturated table), but there's little reason to not take them as troops, especially with how good the Chapter Master on bike is. What do we take then? The Stormtalon is very very hard to argue with for at least one of our slots, especially if you're looking to take Riflemen anyway. Taking one or multiple Stormtalons for that same role helps you free up those valuable elite slots for Sternguard or Legion. Likewise, if speeders or attack bikes suit your fancy or chapter tactics, they're solid enough options to warrant exploring.
And now, dear readers, comes time for me to open the door for something to you. I've got several more pieces for this series, covering the Heavy Support slot, allies matrix (a crossover with my Let's Be Friends! series), extras (including fortifications, Lords of War, and some Forgeworld options), and then Honor the Codex will begin wrapping up with examining some basic tactics and list construction paradigms, including me vetting the list I've finally settled on. As a part of this conclusion, if you, my readers, have a list that you'd like to break down in a guest article, or see me examine, I need to hear from you! Email me through the Rites gmail, or for those of you sent here by my Facebook posts, feel free to send me a message. I've still got a fair amount of material to cover, but the sooner I can begin my analysis of the content you submit, the more steady a stream of articles I can maintain for you.
But otherwise, I'll see you all here shortly for part 8 and the Heavy Support section!