How bad is it? It's a late sequel to a film known for its terribleness.
Should you see it? Sure. It isn't top-shelf, but it has its moments.
More than a decade after Bert Gordon defiled H.G. Wells' story with his movie, someone else decided to cash in on that (?) and make a sequel. The cheesiness is 1980's, rather than 1970's and that's essentially the difference; some people love this film, others hate it and I'm ambivalent. The plot has an experimental growth drug used on tomatoes, which are eaten by rats, which then grow large and vicious. It's slow until an over-the-top gross-out 20 minutes at the end.
There's an eight foot tall toddler that screams obscenities and becomes homicidal. The kills are telegraphed and occasionally meant to be funny (e.g. the guy who stops to pee twice, the playing of "Three Blind Mice"), but there's a lot of unintentional humor. Boom mics and crew members are seen. There's an exterminator with a flame thrower. There's a synchronized swim team massacre, followed by machine-gunning of rodents (and a few people). The special effects are often shoddy. One actor is nearly bald in one scene and has a wig in another.