I'm a psychiatrist in the Army National Guard (ARNG). There will be some differences in the ARNG vs. Army Reserve (AR) thing, but a lot of overlap.
I enjoy my service. I am kept busy. For some physicians in the Reserve Corps (RC), there are complaints about not performing clinical duty. This is not the case for psychiatry. I am constantly doing evaluations of soldiers. Lots of PTSD, substance abuse, and axis II. Lots of evals for fitness for duty, security clearance. Occasional weighing in on psychological autopsy and other once-off issues. As a psychiatrist, I tend to get meatier and more unusual cases, as the more routine psychopathology is handled by behavioral health officers (BHOs, usually LCSWs) who do great work and I function as a de facto team lead a lot and get to conduct lots of training. Duties are pretty similar for Reserve Corps.
One thing you see a lot on these boards is discussion of skills atrophy. This is less of an issue for psychiatrists, and for most of the AMEDD folks from recent deployments (I have not), they've mentioned that the psychiatrist is typically one of the busiest docs. When things are busy and scary, psychopathology presents and you have to deal with it. When things are slow and boring, psychopathology gets noticed and you have to deal with it. Actual sickness gets amplified overseas and malingered sickness becomes attempted more overseas.
The big difference that I see between ARNG that is likely relevant to you is that ARNG is primarily combat units, and Army Reserve is primarily support. The big differences is that with this organization is that ARNG will mostly have you functioning as Division Psychiatrist for an infantry unit or some such (or assigned as such but drilling at state HQ) whereas with the Army Reserve you might be attached to a hospital unit. Not sure how much this affects drill. For deployment, the ARNG usually sends psychiatrists overseas with their home (infantry) unit or as an individual augmentee overseas. From what I've seen, the AR sometimes send the entire unit someplace, but much more common is to be activated as an individual augmentee for a stateside slot. Most of the AR psychiatrists I've talked to have been activated to Fort Hood, Fort Dix, etc., even in peacetime.
I mention this because (warning: editorializing) the Army has gotten used to activating docs/psychiatrists during OEF/OIF and I'd imagine that it is pretty attractive to help mitigate short-staffing stateside on Active side by activating docs/psychiatrists from the AR for 90 days every few years. This may be particularly true for psychiatrists, which is undermanned and there is more attention each year to mental health (and how it affects readiness). As an ARNG, when I get deployed, it's likely going to be for natural disaster in my state or internationally due to war; with AR, when you get deployed, it may be more frequently for stateside missions. Just my $0.02.
There's limited information on military psychiatry on these forums (especially RC). I'm happy to answer more questions.