There are those that have much more experience here on this site and may advise differently but my opinion is that if the pentasa has worked for you (no symptoms/concerns) then I would stick with that and NOT go onto the immuran.
My son is on both, he was pulled off the pentasa approx 3 weeks ago and within a week or so discomfort returned and he is now back on it(despite being on 100mg azathioprine(immuran))
The blood tests will keep an eye on your kidneys and bone marrow function. I guess the main issue would be to avoid anyone with illnesses such as measles, chicken pox etc as obviously the drug will suppress your immune system making you more susceptible to illness.
Side affects can include feeling/being sick, diarrhoea, fever, skin rash, tireness, unexpected bruising/bleeding, chest infection, jaundice, hair loss.
The only problem ds has had really is slight hair loss(don't think he himself has noticed it-I only know cause the hair trap in shower needs to be cleaned out more frequently)