These guys really are the stars of the 5th edition Space Marine codex. Like the Land Raider they are inexorably associated with, they have built up an infamous reputation for assault power, cost efficiency, and above all a massive durability. Still, even these guys are not invincible, and using them correctly will often mean the difference between victory and defeat.
First of all, which ratio of lightning claws to thunder hammers is best? While many armies I've seen run the full squad with TH/SS, I've chosen a single member armed with lightning claws. Combined with my Librarian's Null Zone, this guy gives me the edge over other Terminator Assault squads, as well as putting out some attacks at a higher Initiative which can be really helpful when you're up against I2/3/4 infantry to reduce incoming enemy attacks or at least get some attacks in at the same Initiative.
Stat Strengths and Weaknesses
As Space Marine veterans, Assault Terminators have decent characteristics but no particular high points apart from their armour and invulnerable saves (if equipped with Storm Shields). Their strength in close combat relies on this protection first, and their special weaponry second.
Their Weapon Skill is an average 4 which is actually quite low for an assault specialist. With half of your attacks usually missing, you will want to get the charge off against most targets. Fortunately a (practically mandatory) Land Raider transport should allow you to do just this.
With one Wound each, a few bad rolls on those armour saves can quickly see the squad vanish from the board to sporadic lasgun fire. While this is true for every model in the game, a lot of players consider a 2+/3++ an insurance of survival for at least a couple of turns. This is not so. Some games you may find your Terminators will shrug off an entire army's worth of firepower much to the chagrin of your opponent. At other times they will fail their first three saves and you'll be below half strength before you even get to swing a single thunder hammer. This is the fickle nature of the Dice Gods and the Terminators are subject to them as much as everything else in the game.
Finally, at Leadership 9 (10 with an attached character), unless they're Stubborn they will fail Morale Checks as "easily" as most other Space Marine units. I've had my Assault Terminators scared off the board by a Rhino tank shock and it's not pretty. Try to minimize your opponent's opportunities to force Morale or Pinning checks on these guys because it can effectivily neutralize them from the game (and it's easier than killing them).
As the most proficient close combat unit in your army (arguably in the game as well), the main function of this squad in your force should be as an assault deterrent for your opponent. As long as your enemy can't engage you with multiple powerful assault units of his own he will not want to bring his big hitters in close range. If he has an expensive, fast combat unit he will likely try and keep it away from your Terminators, to the point of keeping it back among his lines, or sending it on a detour which will slow it down. By keeping your Land Raider central in your lines your entire army will benefit from this assault buffer zone.
The second function is as a spearhead for your assault. Keeping the buffer effect mentioned above in mind, you shouldn't send out your Raider ahead of the rest of your force, unless there are no enemy assault units to deter.
It is vital that you don't commit your Terminators too early and risk them getting bogged down fighting grunts while the enemy's monstrous creatures wreack havoc among your Tactical Marines. For instance, when a Storm Boyz squad swoops forward and attacks your lines early in the game, you should consider whether they require your Terminators' expensive attention or can be dealt with by other units. However, if Nob Bikerz come knocking on your Rhinos' doors it's probably best to send out the Termies.
This brings us to the matter of target selection...
Targets of choice
The main strength of your Assault Terminators comes from the protection of storm shields against incoming power attacks. Keeping that in mind they are the rock to enemy Monstrous Creatures' scissors. Low number of attacks will keep your Terminators alive where other troops are crushed without hope of a save. Your high number of thunder hammer attacks should see most Monstrous Creatures crushed in one or two turns.
Two may take a bit longer
Secondly, S8 means you can instant-kill most multiple-wound units, like Nobs or Tyranid Warriors. However, these units will usually strike before you so thinning them out a little with ranged weapons, and cleaning up the remainder with your Terminators is best. This is especially the case with Nobz, since most Ork players remove Big Choppaz first when taking casualties, leaving the Powerklawz for your Terminators to slay. Using a Vindicator to thin the enemy's numbers works very well, and should you scatter horribly your Terminators can shrug off most hits they might take.
Succes not always guaranteed
Targets of opportunity
When the enemy possesses no big monsters or Nob-like units, you will need to find some weaker unit for your Terminators to smash to a pulp. Against such armies your Terminators are more of a distraction that can be sacrificed as the rest of your army can handle your opponent's units as well. As such, going after your enemy's scoring units is usually a good choice. Big squads with few attacks are best, so Tactical Squads, Dire Avengers, Necron Warriors are all good choices.
Secondly, expensive vehicles such as Leman Russes, Defilers or Predators are juicy targets your Terminators will be able to destroy quickly in close combat. With the high number of attacks a full squad puts out, even a vehicle moving at cruising speed won't be safe.
Popping independent characters can also be worthwhile, especially if they're vulnerable to Instant Death.
Targets to avoid
Storm shields do not protect against regular attacks and while a 2+ save will ward off some blows, it won't stop them all. With this in mind, large squads that put out lots of attacks are best left alone. Ork Boy mobz, Howling Banshees and Genestealers are best dealt with by your Tactical Squads and supporting units, as they can overpower your Terminators through weight of numbers.
The Emperor did not protect
You should be wary of these squads as a clever opponent will try and catch your Terminators in combat with these units when they're out and about smashing his more valuable units. Using Land Speeders or Rhinos to shield your Terminators from counter-charges while they're engaged in combat can protect them for a turn. Alternatively, you can send out a Tactical or Assault squad in combat alongside the Terminators to absorb some wounds.
If your Terminators have done their job, however, don't go to great lengths to save them from destruction. They are not scoring units and can be sacrificed in most games without diminishing your chances for victory.