Blastmasters, blastmasters. I love them. They fill me with joy, the likes only a slannesh daemon prince has experienced. As you can imagine, this is a lot of joy
48"
str 8
ap 3 small blast, or assualt 2, 36" str5 ap4, both ignore of them ignore cover.
IMO shooting this gun at something that is
AP 2 is usually a waste, not always, but normally, which is why I am primarily using it to punch huge holes into enemy infantry. You can make mince meat of a wide variety of guys. Most things without invul saves, or high T
fnp get rekt. My friend plays tau and being able to out range his firewarrior squads while ripping holes in crisis battle suits is nice. Forcing leadership tests are also a nice bonus of crumping little squads. Its also great against bikes and jetbikes, and can be used in a pinch against skimmer based tanks like necrons and eldar. I personally like to take a sicaran or 2 in my list as way to deal with light vehicles to leave my BM for shooting at infantry, but they can be used nicely in a pinch.
My meta consists of eldar, tau, necron and tyranids. I play
CSM and Orks

Ive also never been to a tourny or any real competitive event, so take what I say with a grain of salt of course,. I try to take as many BMs as I can if that is the choice for anti infantry/light vehicles that I am going for. In order for them to work you need to have good target saturation. They have 48" range which is outside the range of small arms meaning your enemy will have to divert big guns to shoot at this unit. The
faq allowing 5man BM squads is nice so I put one in a 5man squad for 125 points nice and cheap (for
CSM that is) . Also, the Noise Marines are fearless, so you essentially have a 5 wound BM unit that wont run away
The sonic dread is also pretty kickass if not expensive, but for 190 you can get 2 BM shots per turn with it, as long as it does not move that turn. Also has rending+gets hot with selective activation as a free upgrade. The sonic dreads
BS is 5 also so the shots are pretty accurate. The combo of dread(s)+ small squads of BM marines can provide a really pretty scary kill range, and with good deployment, you can might be able to get a couple of free shots before things close in.
Of course, with all of this anti infantry, you are going to need a way to knock out flyers and tanks, so this list is going to need good anti tank compatibility but as far as gunline anti infantry
CSM, I love the blastmaster.
Here is an example list of what I can run:
Lord Slannesh
Sorc
2x sonic dread. bm warp
amp
4x5 noise rines BM
3x5 raptors 2x melta combi melta
2xrelic sicaran
2x oblits
1845
pretty standard here, deploy far away, let he BMs and sicarans do work, deep strike the meltas and oblits, cross fingers and pray to slannesh. If I am going against my tyranid friend, or thought flyers were coming my way, I would take a fire raptor for some
AA 
. Sorc and lord are barebones for tax. In fun lists I like to deck them out, but this is a "more serious" list.
On a completely -cough- unrelated topic

I recently just ordered some 2nd edition noise marines from ebay to use as my blastmaster models in my squads. The old models look way more badass
imo. Cant wait for them to show up. Also I was lucky enough to score a couple sonic dreads for cheap so this might influence my bias a bit for my love of the BM
Coming from a long time ork and
CSM player its definitely one of the best guns from those 2 codices so its probably why I enjoy using it so much against my tau and necron friends haha
So, that being said, how do you guys run your blastmasters? How do you support them? Why or why do you not choose to use them?
If you made it this far

thanks for reading!