I'll be honest with you, after finishing Brotherhood in a quarantine marathon, I was a little biased against 03 at first due to being a diehard fan of the original manga a while ago, and more recently, adoring Brotherhood. Nonetheless, I can't say I regret picking it up.
For the positives, I think my favorite part of 03 is the world building it had that Brotherhood lacked. I truly had a lot of fun watching the "filler" parts. The standalone episodes really gave you a closer look of the one-time shenanigans the two brothers got themselves into during their travels, an element that made me fall in love with other iconic anime like Samurai Champloo. Not to mention, earlier arcs parallel to that of Brotherhood's were more drawn out, notably the Leto and Tucker arcs. Even the Yoki arc got an actual animated adaptation as opposed to Brotherhood's quick recap several seasons in. Martel (or Marta in the dub, but Ed's controversially unsavory voice actor steered me away from it.), who had her screen time whisked away in Brotherhood, got more time to shine. So ironically enough, some smaller elements from the manga that Brotherhood breezed over were captured perfectly in 03.
Now to the negatives, I can't say I was a huge fan of the central plot and its twists. The story changes just weren't my thing. In fact, its what tanked this entire review to a 7.8/1o. I won't go too far into it, as I'm trying to keep this spoiler free, but I gotta say, I felt as though it could've been developed more, and just felt weird near the end. By 40, I found myself thinking "uh oh... Only 11 more eps in a 51 ep series?" and the ending left me scratching my head. I suppose I'm left with no choice but to watch Shamballa now.
So the final verdict? Definitely give this one a shot if you're a fan of the manga or Brotherhood, but be prepared for some MAJOR plot divergences.