Marine codex 'review' (#4): troops.

Tactical squad: bad.

People love stacking these, and then whine they can't win. The first mistake of all new marine players: building their armies around tactical squads.
As we have touched upon already, marine armies don't have 'cores,' or 'supporting elements.' Your grunts are just as self-contained as your elites, and aren't on the table to act as the core, support, meatshield, or anything but their own units.

In addition to their cost, tactical squads have a couple problems. If you want them to do anything, you need ten men. If you have ten men, you can't fit the unit and a character inside a rhino, or even use the full unit effectively with a razorback. Once you add up the cost of bling, gear, the five extra men, and your ride, you'll come out at least at 205 points. That's a lot for what most consider a glorified scoring unit.

In a balanced 2000 points army, you aren't likely to ever need more than two full tactical squads. In fact, all marine armies not built around a biker boss or massed razorbacks NEED two full squads.
However, just as there is a law of conservation of ninjutsu, there is also a law of conservation of full tacs. Essentially, there is only so much power inherent in full tactical squads, but what there is ends up split evenly between all squads present. Therefore, the less of them you take, the stronger your individual squads will be.

Note that while tactical squads are bad, being 'bad' is very far away from being 'useless.'

Your stats and gear let you soak and tank quite a bit of punishment, and the squad is good at causing heavy real-estate damage to enemy light infantry.
Most of the truly good stuff is 'free:' multi-melta, flamer, missile launcher. Putting a multi-melta in both your squads is almost mandatory, because it provides an invisible forcefield aura that tells enemy armor to stay more than 24 inches away, and should any vehicle try and tankshock you, it's likely to get slagged right then and there.

For added melta goodness, you can equip the sergeant with a combi-melta. If you're giving him anything at all, always give him a combi-melta in addition to his other gear. It makes your unit that much more useful to you.
It's sometimes worth it to upgrade your free flamer to a meltagun, too. Remember, this increases your anti-armor capabilites with a factor equal to the anti-infantry you give up.

Another option, which is a lot less useful, more expensive, but far more popular, is massed plasma. Combi-plasma, plasmagun, plasmacannon. Not really a wise thing to run in 5th edition.

The popular consenus is that tactical squads suck - which they honestly don't.
It's a limited unit, yes, but it's powerful enough, reliable enough, and flexible enough to work on its own alongside your other mounted infantry and armor, and in addition to this, it's a scoring unit. This is someting that cannot be ignored.

Tactical squads will never measure up to troops such as grey hunters, imperial guard veterans, most tyranid troops, or even plague marines, but those units don't have the pleasure of being self-contained, or even half as reliable as what you bring.

That said, unless you want more plasmabacks or assbacks, you'll get a lot more bang for your buck from other units.
Don't sink more than the necessary points into tactical squads - full ones, especially.

What makes it good: it's a scoring unit. Always brings armor saturation. Free multi-melta scares enemy armor into submission, bolters plus flamer and/or combi-flamer tear chunks out of light infantry, and extra melta weapons allow you to kill armor quickly and effectively.

What makes it bad: expensive for what you get. Because of the fixed 'one special, one heavy,' you're likely to end up with one marine shooting his pistol, and the other using a support weapon. Heavy weapons in tactical squads are more for sniping than active use.

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Scouts: bad.

These were formerly very good, but are now a whole lot less so.
With a significantly weaker profile than their big brothers, scouts don't really do anything that well, and unlike tactical marines, they don't bring a big box of tools, or a transport. Even their starting gear is weaker - trading power armor for carapace.

This is all really bad, but there are redeeming qualities.
Instead of a toolbox, scouts have a huge list of handy special abilities. Like you'd expect from the name, scouts are indeed scouts, and bring both infiltrate and move through cover.
You can gear them up with shotguns, bolters, sniper rifles, or really large knives, and it won't cost you a single thing.
Shotguns make your scouts into short-ranged dakka units. In essence, the kids are pretending to be sternguard. Knives let you pretend you're an effective combat unit, and bolters are available everywhere else in the army, too. It's all really fluffy and cuddly, but not worth your time.
Another option rarely worth your time is 'camo cloaks.' This gives the unit the stealth rule. You can also splash points for inaccurate missiles, or 'hellfire shells' heavy bolters. Both should mostly be avoided.

Then there's the rifle.
Note that scouts are the only available platform in your army for these.
Sniper rifles give you disruption and long range. Sure, they still won't hit much, but infiltrate them close to an unsupported flank, scout move them into some terrain, and they'll sit there for the entire game, killing or pinning the occasional small unit, plus probably rending a wound or two from a monster.

Unlike other books' disruption, yours doesn't automatically die from regular flamers, and can fight the small units usually sent in to flush disruption out.
Also quite unlike most other disruption, yours are troops, and thus scoring units. This puts pressure on many players, and even allows you to secure homebase, while both tactical squads go on the offense.

While they're not killy, at all, they're still capable of beating enemy support units in combat. Lootas, devastator equivalents, heavy weapon squads, and etc. all fall to your scouts' rifle buttstocks, knives, and pistols. If you bring scouts, take advantage of this fact.

In other words, even your bad units bring something useful to the party, and should you ever feel that your two mandatory tactical squads aren't quite enough - or you desire disruption - add some scouts.

What makes it good: it's a scoring unit. Fairly priced disruption, all things considered. Some armies have problems getting multiple small scout squads off objectives. Rifles let you rend wounds off monsters, and occasionally, you'll even pin a squad. Despite junky stats, still a combat threat to the support units fielded by many races.

What makes it bad: not killy at all. Utterly useless against anything with an armor value. Fears AP4.