cheap jigsaws are only any good for cutting up to about 18mm thick really. Any thicker and the blade will wander and the cut wont be at right angles. Also you will need to keep changing blades as they blunt. Jigsaws and any type of straight edge guide is a waste of time as again the blade will wander despite using an edge guide. BUT jigsaws are safe to use whereas circular saws less so - you can do alot of damage with these things ( I hate them and I've worked with wood for 40yrs) If you building cabinetry in a room etc you'll need to cut presume softwood batten or studding ie 3x2 type of thing also plywood? MDF sheet? etc. Go buy 2 handsaws for a tenner from BQ etc - they will be razor sharp to start with and cut through materials like butter. The biggest tip is to use a candle and rub in over the saw blade as this stops the the saw jamming when making a cut.