hi kristya! i also take a combo of the xr and the ir adderall. i think i started sometime in september? i have had similar experiences, though only one incidence of dizziness…and that had happened to me before – way before taking any meds. overall my experience is that it is helpful, sometimes more sometimes less. if i haven’t had adequate sleep or am very stressed(more than usual) it can feel like it’s not working at all.
had an experience last month that really shook me. had been feeling more groggy than usual for a few days, also more scattered and all, and then one morning i was so tired that when i got home after dropping my son to school i konked out on the couch i think as soon as i hit it. didn’t take my morning med at the ususal time so i could sleep in, and didn’t have alarms set because i wasn’t working that day. i woke a few hours later, still super tired, but took my med hoping it would help me function. i promptly fell back asleep…..and didn’t wake again until 5 pm! and only then because husband had just returned with our 2 boisterous kids. i took my ir then, even though it was late, hoping to be alert and together enough to help my son with his homework. i had a bite to eat, made sure his homework was complete, got him ready for bed…..and went back to sleep! the next day , though i didn’t pass out all day again, i still felt tired and “out of it” all day and night, despite taking my meds. the following day i took extra med(more ir) because i needed to be alert (didn’t trust that the other parents would be adequately vigilent about monitoring the kids in the pool) and i was ok. actually by the evening i felt better than i had in days.
and then i began to really worry – had i developed a tolerance to the med already? is that even possible in so short a period of time?
well, i told my psych about what happened. he said definitely not a tolerance issue, but sounded more like a prodrome of a bad flu. sure enough, days later – after caring for my son who was sick with some sort of upper respiratory bug, it hit me, and hit me hard. and while it was quite uncomfortable at times, i was relieved that it wasn’t an issue with the med.
the point being, the med can only do so much, and it’s effects are very much affected by your circumstances – sleep/health/stress/diet/…
also, from what i know, the active chemical components-including the isomers- are the same for the xr and ir, it’s just the xr is coated for different time release in order to achieve longer and more level blood levels. just the same, i also feel i do better with the ir, i’m guessing probably for some period the blood levels go higher than with the xr that i’m taking, and think other factors may influence this too….which may mean i’m not on an ideal dose of the xr, but don’t really want to go higher with the xr if i can avoid it, and also don’t really want to have to worry about remembering to take meds even more often than i do now if i switch to all ir 
i don’t know if this helps, but at least you know you’re not alone 