I really like Laserburn and Imperial Commander. Laserburn works best with around 12-20 figures per side, certainly not more than that. Imperial Commander works with 50+ figures per side and is designed as a larger skirmish set than Laserburn.
Laserburn is slightly clunky by modern standards certainly, but I would not say that it is "not up to" them. There are plenty of badly designed modern rules sets that are clunky and unwieldy or just give a poor game.
Laserburn will certainly test your maths skills a bit since it uses a system of deterioration of accuracy rather than range bands. This means that if you fire your Laser Rifle at someone, you deduct 1 from your chance to hit for each 1cm range, while a Bolt Rifle deducts 2. There are quite a few other modifiers to work through too, but you soon come to terms with them if you play the system. Overall, it may not be as streamlined as some systems but it works well enough and gives an enjoyable game that is enhanced by the introductory text, which includes suggestions for organ-legging scenarios (For a Few Gonads More) amongst other things.
I have enjoyed Laserburn and Imperial Commander since the early eighties and still do. I also enjoy more modern game systems like 5150 and Urban War, each of which gives me a different gaming experience. I also think that the Beamstrike project which seeks to produce a Laserburn-like game is well worth checking out too. A lot of hard work has gone into that project and I think it has paid off.