I'm torn, personally.
On one hand, it's quite beautiful for its hardware, as many have noted. The music's mighty catchy, when one can make it out over the deafening sound effects of asplozhuns and stuff. On the other hand, those aesthetics and multilayered levels work against it when the so-tall-it-scrolls vertical playing field obscures very real threats just offscreen (See the above-water water and desert stages out of the initial rotation for an example). Perhaps a candidate for a TATE remake?
Gameplay's intense when done it's right: holy-smokes-there's-so-many-targets-and-theyjustwontDIEDIEDIESWEETBEJEEBUS! However, as others have beat me to the punch on, dying and losing decent weapons before those super-resilient bosses makes for one heck of a fight. Personally, I like my bosses beefy, with intensifying odds against me, and so I found those pea-shooter experiences bizarrely more satisfying than the "park-in-the-corner-with-the-Hunter" method that was so much more effective (and of course, required specific stage selection and craftiness to gain and retain). Still, dropping three or four lives at a pop on a boss due to iffiness on where those moves are going and weird movement/hit detection/something were common experiences.
The mega-laser doodad from level 6 onward is most amusing, but creates an imbalance in your favor when you've got it, and an imbalance in the enemy's favor when they deprive you of it. Makes the difference between a ten-second or an eight-minute boss battle.
That of course, views the game in a bubble on its own. Compared to TF3, TF4 has more stuff, more beauty, and more variety. TF5, well, TF4's just got that different vibe. Tough to place. But, anyway.
And so, it's a flawed, fun one for a burst of variety from the top-down stuff that's so popular today. Y'know, that Axelay comparison is eerily accurate. Hah.
In my defense (or is it offense against me? Ha!) I haven't made it through on the Maniac mode. I was plenty satisfied with my month of effort finangling Normal. Are there any significant changes gameplay-wise that could influence this fuddy-duddy's opinion? Because everyone does know that Axelay doesn't really pick up until the loop AFTER Hard mode, now, and so might this.