I've been using IM+ for several years, and I have no complaints with it. It is, however, more expensive than Jivetalk. If Jive had been available when I bought IM+, I probably would have taken the less costly approach.
SMS is text messages. Some Instant Message programs use SMS, as opposed to DATA, which applies against your BlackBerry Data Plan, the kilobytes or megabytes of data you use on the BB, or unlimited plans.