I've never thought I'd like a custom character so much after I happened to find Dirk on a server. He's so consistent and charismatic design-wise and his abilities make him so versatile it baffles me that SRB2 has been missing him for so long.
Since I've had my share of experience with him in some Multiplayer game modes, I guess it wouldn't hurt to contribute, starting with the fact that Dirk is a complete - too complete - character and thus... pretty much "broken" in Competition and Circuit.
Bodies of water are an almost universal challenge in SRB2 for vanilla characters as much as for most of the custom characters but Dirk is able to almost completely brush them off by simply running over them when convenient, maybe second only to Tails, who can fly over them (and out of them when not completely submerged), but will eventually fall into them if the length excedes their flight span.
Vertical variation, which can be overcome by Tails and Knuckles but only with extreme time penalties, are almost nothing for Dirk with virtually no loss in momentum or pace, and while he might not be a match to Sonic in horizontal variation (which I don't believe) Sonic loses to him in almost every other aspect. Picking Dirk with enough practice in every Coop and Circuit map will almost certainly give him the victory.
Of course, that doesn't mean the problem is in Dirk himself. Most Circuit maps and the vanilla Coop rotation are made thinking of the vanilla characters and thus most of them aren't a challenge to Dirk and his solid set of abilities and their possible combinations - which is why I think Dirk deserves levels made for him, thinking of how he can use his full potential. At the moment I can't conceive a way to bring the vanilla characters to Dirk's level in terms of skill sets, and the most I can do is congratulate you for such a creative character.
Oh, yeah, there are still a few problems with him - those being the ice trail. It's too long, takes too much time to disappear, and it's solid, meaning it's a real obstacle if any other player is close to Dirk - players can be knocked off track by it, bump into it, be slowed down by it, walk on it if running behind Dirk and eventually die from falling into a pit when it finally disappears, which is really annoying if we take into account how unbalanced he already is, and the fact that he can keep walking on water even if he loses speed or stops completely by bumping into a wall, although I think this was not intended. In fact, reducing any of those variables (trail length, trail duration, solidness) would already do him great service.
And finally, a suggestion - his ice ring really feels out of place in comparison to every other character's normal shooting. It doesn't look like he can kill or even hurt other players with it, so it's a huge handicap that can't be coped with by his mobility.
To not scrap the idea entirely, we could have Dirk shoot with normal red rings, but if he ever feels like he needs to just freeze someone, have him charge his rings with ice (by a button of choice) using a chunk of the same bar he uses for ice-skating, freezing his opponents and hurting them when the ice breaks, Ice Cap style. In the same way, Dirk could press a button and freeze himself into a block of ice, making himself invulnerable to damage for a short time, but unable to move, and could do the same to his allies. The blocks could slide and be pushable, so he could throw or be thrown in a pit... There a lot of things to explore.
But then again, I've talked for too long. Just wanted you to know how much I appreciate and grew fond of this character, and of course, want to see him being given the love he deserves. Keep doing a great job.