I just want to spend a little time talking about my favourite unit in the Space Marine Codex; the Thunderfire Cannon (TFC). Now this little piece of artillery comes in for a fair bit of flak on the interwebz but I wonder how many of these people have actually ever used on in anger?
I completely appreciate the fact that it's an AV10 piece of artillery, i.e. a glancing hit will destroy it, no roll on the damage table necessary. Yes, I know this means it can be destroyed by small arms fire, and yes I know it's a completely stationary fire-base. So on paper it's pretty easy to say that this particular 100pt's of your list could be better spent almost anywhere else.
However I'm here to tell you that I've used the thing in two tournaments this year and it's been bloody amazing in both. In my opinion, as a Marine player this is the only way to reliably counter a true horde army (two would be better!). Against Tyranids, Daemons and Orks it really comes into it's own because they don't have the long range fire power to spare to take it out early enough for it not to make an impact, and really it can earn it's points back in one turn of shooting.
Against armies which rely on movement it has shells which cause the unit you hit (only need to hit, not wound) to be in difficult and dangerous terrain. This can be crippling to some armies, likewise those that rely on using cover which is so easy to use in 5th, the TFC can ignore cover and easily descimate those weaker units.
So what about the fact that 5th is all about Mech and MEQs are always prevalent? Well, vs Mech the TFC is not optimal but by putting one into my list I can go overboard on anti-mech units in the rest of my list because I know I've got an anti-infantry beast in my back-lines. Also there's the oft quoted stat that you are more likely to immobilise at Land Raider with Earth-Shaker Shells than you are with a Lascannon: with 4 blasts you are statistically guaranteed to get a hit with the TFC then it's a 1/6 chance of immobilisation if your opponent decides to move it next turn.
I could go on further but if you've never used one then I'm not going to change your mind but remember that 40k is not played on paper, it's a table-top wargame and you're playing against another person, not statistics.