i have to say some employers are cool with it. with that said you must do your part in informing your employer that you have to attend drill and AT well in advance and any changes that will come up they always do. the are required by USERRA if i spelled that correctly to give you time off. they are however not required to pay you.
it does get hard though taking off from work. it can be done. all depends on your job really and what you do. if you work only mon thru fri then weekends are no problem. if you work say retail and your hours are all over the place. inform your employer.
it is against federal law to be discriminated for being in the Reserves or Guard. but an employer will not tell you outright. but you can tell when they do. it has been done to me many times. when Iraq was going off i could'nt find a job to save my life. so i work for the Army as a civilian and we are all Reservists and most have been deployed once or twice. so no issues at my job.
good luck to you i hope this may help. there are more here who may know more than i do.