I don't know if I would go so far as to call them great. They are a good band, but very little other than Petrucci's playing is what I would call excellent. The vocals are good, but they're not David Bowie or anything. The piano is amazing, but great? I think Thelonius Monk, Vince Guaraldi, or Ray Charles when I think of piano greats. Mike Portnoy is definitely above average, but Keith Moon he is not. Great bands are pretty rare, and usually they're great because they either A. Invented something new (Pink Floyd), or B. Wrote songs in their genre clearly above and beyond the others (Bach).