I played my first game this afternoon using a 20 man Primaris Crusader Squad versus Chaos Space Marines:
8x Neophyte - Bolt Pistol & Chainsword
4x Initiate - Heavy Bolt Pistol & Powerfist
7x Initiate - Heavy Bolt Pistol & Chainsword
Sword Brethren - Heavy Bolt Pistol & Power Axe
I selected Helbrecht and Grimaldus as my two HQ, mainly to buff the squad - +1S, 6s to wound in melee converted to mortal wounds, and 5+++ Feel No Pain. I selected the popular vow for 5++ invulnerable save, and mini Transhuman (cannot be wounded on a roll of 1 or 2).
The squad did well enough, quickly cutting through a large cultist squad and then tying up a Lord Discordant until my terminators could crush the metal beast. By the end of the game the squad was down to around eight Initiates, so they held up rather well. I will definitely use them again.
Positioning is very important. The first two turns I tended to pull models from the front when removing casualties, and then later pulled them from the back, keeping all four powerfists up front so they could punch. You’ll probably never get the entire squad in engagement range but obviously they are very good at holding and stealing primary objectives. Pro Tip - use the Devout Push stratagem once they have charged to get more models in engagement range !