i don't like jems, but am very familiar with basswood and alder. my main guitars are made of alder, it's a very well balanced tone wood. i've always considered alder a solid choice for a body wood, nothing special about it, but nothing bad about it either.
basswood gets a bad rap, probably because it's cheap (price-wise) and plentiful, especially in japan. i'm guessing when the japanese started using this wood to make guitars, american guitar builders used this as an argument against japanese guitars, asserting that the wood was 'inferior'. nowadays, basswood is used in all levels of guitars from the high end (jems, suhrs, js
models, evh ernie balls) to the bottom of the rack entry level guitars. i've heard that there is a tonal difference in basswood depending on what part of the tree the wood comes from and that there is a desirable basswood and an undesirable cut of basswood. dunno if that is true, but from my experience, basswood emphasizes mids and low end with a nice rounded off high end. one quality of the wood that is very desirable for soloist is a somewhat compressed nature to the tone. basswood has plenty of low end, but it is a soft 'spongy' low end which isn't always suitable for tight distorted palm muting rhythm guitar.
although my main guitars are alder, i love basswood. my js1000 & axis sound incredible, better than my alder guitars...