Spell Familiars are insanely good. The normal chance to cast is here, but in Red is the chance to cast with a spell familiar.
Chance of successful cast for a 1 Warp Charge spell:
1d6: 50% 75%
2d6: 75% 93.75%
3d6: 87.5% 98.4%
4d6: 93.75% 99.6%
5d6: 96.88% 99.9%
6d6: 98.44% 99.9%
Casting with 1 dice now becomes viable, casting on 2 seems to be almost guaranteed.
Chance of successful cast for a 2 Warp Charge spell:
2d6: 25% 43.75%
3d6: 50% 75%
4d6: 68.75% 90.2%
5d6: 81.25% 96.4%
6d6: 89.06% 98.8%
7d6: 93.75% 99.6%
8d6: 96.48% 99.8%
Casting with 3 dice now becomes viable, casting on 4 seems to be almost guaranteed.
Chance of successful cast for a 3 Warp Charge spell:
3d6: 12.50% 23.4%
4d6: 31.25% 52.7%
5d6: 50.00% 75%
6d6: 65.63% 88.1%
7d6: 77.34% 94.8%
8d6: 85.55% 97.9%
9d6: 91.02% 99.1%
Casting with 4 dice now becomes almost the same as without using 5 (normally considered good), and 5 is better than 6 (normally considered viable), casting on 6 seems to be almost guaranteed.
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A lot of stuff it is about finding what you have and finding out where your weaknesses are and try to prevent them.
Personally I love Maulerfiends running along side bikers and chaos spawns. Also if unbound things like Hounds of Khorne from Daemons are nice and cheep and normally a bit of a threat.