To tell you the truth, I once upon a time held a (largely petty) bias against this group, even before I'd ever heard a single tune of theirs (even the über-famous A Whiter Shade of Pale). You see, back when the Moody Blues were the center of my musical universe, I happened across the (incredibly negative) Rolling Stone music reviews for the Moodies, and I wanted both to cry my eyes out and to punch a hole in my computer screen. One of the things that stood out in their slamming of my (then favorite) band was that, while presenting the Moody Blues as an example of a "bad" fusion of rock and symphonic ideas, they held up Procol Harum as, at least, a "decent" fusion of rock and symphonic ideas. Well, this would simply not do, and I decided in advance never to listen to a single Procol Harum album as long as I lived. Fortunately, I grew up sufficiently so as to not be so moronically close-minded, and I soon discovered that Procol Harum was, indeed, a most worthy band to add to my listening experience.
Still, even though my musical tastes adapted in such a way that my total adoration for the Moodies was tempered at least a bit while my adoration for Procol Harum grew, I found that I continued to regard Procol Harum a bit lower than the Moodies (if you're a hardcore fan angry with me for continually bringing up the Moodies, I apologize - I really do like this band, as you'll see later. It just so happens that comparisons with the Moody Blues are justified, as they're more than a bit alike). Mostly, I think this is due to one thing - Gary Brooker, as hard as he tried, could never ever measure up to any of the Moody quartet in terms of vocal resonance and power. I honestly cannot think of a single instance in the Procol Harum catalogue where I can say that Brooker's singing is a "deciding factor" in turning a track from a decent song to a good one or a good one to a great one - in contrast, the Moodies' "group harmonies" still run chills down my back, and it goes without saying that Justin Hayward, John Lodge, Ray Thomas and Mike Pinder have, at some time or another, significantly impacted my mood solely by the sound of their individual voices.
As a corollary, the oft-lauded poetry of lyricist Keith Reid is, to an extent, wasted on a pleb like me. Now don't get me wrong - this guy is good at what he does, and the times where I've looked for Procol Harum lyrics on the net (off-topic rant: it really bugs me that, for all the importance placed on Reid's lyrics, the only Procol CDs I have that contain a lyrics sheet are Something Magic and The Prodigal Stranger, incidentally the two worst offerings in their catalogue) have often left me marvelling at what an elegant poet he can be. Unfortunately, as resonant his lyrics may be as poetry, they lose much of their impact on me when filtered through Gary's voice - unless I'm really paying attention to the lyrics, they end up consigned to part of the background for me, merely an extra layer in the overall sound. I'm terribly sorry, but that's the sad truth for me - if the lyrics substantially move you when you listen to Procol, then more power to you, and I envy you. I ask only that you not hate me for my "failing" as it were.
THAT SAID, the fact that I don't find resonance in the vocals and lyrics does not mean I find no resonance in Procol Harum as a whole. He could be better at some times, a smidge worse at others, but Gary's weakness in singing was often MORE than made up for in his strength as a composer. Simply put, Gary Brooker had a rare gift, an ability to (often seamlessly) interweave heavy classical elements with "normal" rock and pop, all the while remembering to include solid pop hooks so as to keep the listener entertained. Furthermore, he was not afraid to lighten the mood with some silliness, nor he was he averse to occasionally penning hard rock tunes that could compete with most anybody. He simply knew how to write good songs, ones with untrivial chord changes and riffs, and he knew how to write melodies (both vocal and instrumental) that were plenty resonant on their own, even without Keith's poetry.
What truly sets Procol Harum apart, though, is the sound. Whatever may be, NOBODY else ever had a sound like this, one so deeply rich and heartfelt and all good things like that. In addition to Brooker, an excellent keyboardist in addition to his position as vocalist, the two main "pillars" of the band are keyboardist Matthew Fisher (who also occasionally contributed some brilliant compositions before he left in '69) and guitarist Robin Trower (who left after Broken Barricades). A common trick with the band was for Brooker to stick to piano while Fisher stuck to organ, and the combination of them provided a THICK keyboard sound unmatched in the rock world. The icing on the cake, though, was Trower, often derided as a mere "Hendrix imitator" but an excellent guitarist in his own right. A master of all sorts of cool, heavy guitar tones, and a very skilled and intelligent soloist and riffer, he really provided the ROCK in Harum's brand of art-rock, before going off to pursue his own solo career. As for the other members (Dave Knights on bass, at least for a while; BJ Wilson on drums), well, they're decent enough - nothing spectacular, but nothing bad either.
Anyway, when all is said and done, the band aren't really essential unless you're a big art-rock fan, but this was still a very good, very solid band throughout its life (until their, um, less-than-great comeback in '91). They only made one "truly" great album (ie one that I'd put in my overall top 100) in their career (in my opinion), but they had the good fortune to make quite a few very very good albums, all of which are quite recommendable.