2.5/5 stars. Rounded down. Slow, but has its place.
Background.
Employed only sparingly during the Great Crusade to breach enemy positions, the Mastodon saw increased use in the days of the Heresy as a fully armed and armoured transport capable of protecting its transported marines even against titans to a reasonable degree.
Strengths.
AV=14 all round with 20 hull points and a whopping transport capacity of 42 makes the Mastodon impressive. Only the Sokar Stormbird can compare.
You can carry 4 models that lack infantry or paragon, but can't have greater than bulky of 6 (so yes, you can carry some contemptor dreadnoughts which is pretty unique).
Two void shields help keep it all alive nicely. The siege melta array is nice enough with a large blob at S=10 but obviously short range, and the turret skyreaper is fine with S=7 and skyfire.
Weaknesses.
The movement rate of 10 is a problem for a transport, unless you don't mind moving slowly, which admittedly many people might.
This is the sort of unit that you need to build around. Not only your army list, but also your battleplan.
Builds.
Mastodon, las cannon sponsons, heavy flamer sponsons (620points).
We have to take two lots of sponsons, so the selection here corresponds to what is sold (I acknowledge you get all variants) but its always pictured with this combination and I don't see any reason to disfavour it.

