I love the BJ-1: With the AC20 and 3 ML it just blows things up. I use a 235XL engine in it, with the jump jets you can spread out the damage to your legs as well as the torsos, and you're pretty fast (93kph with speed tweak) so you get missed a lot as well. It takes some finesse, you can't run in and brawl like a heavy, but if you use cover (terrain and allies, it's awesome to poptart over an Atlas for the kill,) and are patient you can do a ton of damage and get a lot of kills.
Before I went with the AC20, I was playing it with two AC2s and 2 Medium Lasers: That was a good build for long range support. I generally did more damage (but got fewer kills) than I did on my BJ-3 with dual PPCs at the time, but my K/D and W/L ratios were better because it encouraged a more conservative style of play. (It also helped me improve my accuracy so I got a lot better when playing my BJ-3 after I had spent the time to get all the basics on my BJ-1.) You can get a good number of kills with the dual AC2 setup, but you have to target your enemies, find the weak spots, and then actually hit them: It takes forever and burns through way too much ammo to work your way through an Atlas's pristine CT, but if the right leg armor is red, you can cripple it really quickly.
My favorite Blackjack is the
BJ-3 with dual PPCs. It's a solid medium ranged support mech, works well as a jump sniper and can brawl pretty well too, as long as it can get enough separation to use the PPCs. Putting both PPCs in one arm is dangerous since you can lose most of your firepower at once, but the convergence is so tight that it's like shooting one weapon. You can pop over a hill, take a pot shot 800 meters away, and both PPCs will hit the same spot every time. I've run in this configuration for well over a hundred games and only remember getting an arm knocked off once, BJ arms are pretty tiny and you have to really aim for them to knock them off, and people don't usually realize that. Usually it's torsos or legs that go, and it's not uncommon to have no armor at all left.
I want to like the BJ-1X, it's fast and you can put 8ML on it with a standard engine. But it's slower than a Cicada or a Jenner and lacks jump jets. Added to the fact that the lasers are damage over time compared to PPCs or ACs, it ends up being less survivable and doing less damage than either of the other two chassis for me. I keep playing it, because I think it will teach me not to overextend and put myself into bad positions, and it will force me to improve my gunnery skills, but I don't think it will ever be one of my favorite mechs the way BJ-1 and BJ-3 are.