I just finished this a few days ago, and overall I really enjoyed it.
I've been a Suikoden fanboy since I and have played every game since, and I fall into that category that thought IV sucked and II was the best (followed closely by III). So going into Tierkreis and hearing a lot of complaints about it not feeling like a Suikoden game had me worried. But by the end, I think I can easily say this does feel like one, even if it's missing a few staple elements.
Story: While not filled with as many shades of gray as past Suikoden games, the story in Tierkreis was still one of the more enjoyable ones in recent memory. There are some low points, but the ideas put forward, some of the plot twists and many of the characters made for an enjoyable experience. The bad guys still have more complex motives than most RPGs and some of the bad guys turn out not even to be that bad after all in the end. The game as a whole does feel a little more lighthearted than others, but there's some pretty heavy stuff mixed in as well. Perhaps people feel it's not a Suikoden game simply because it takes place in another world, but I think Konami did a good job of creating this new world and coming up with some interesting concepts to explore.
Music: Nothing mind blowing, but the music also had the Suikoden flavor to it. Esp. in the war room scenes. In many places the music was very good and I was satisfied with it, though I wouldn't be seeking out an OST.
Gameplay: Maybe that would be my biggest gripe about the game. The game is fun and I still hold to it feeling like Suikoden game in this regard, even with the missing stuff. You get a castle, you collect the 108 stars, and though altered the dynamics of your HQ still feel much the same. The mission system really worked well I thought and essentially replaced the role of the detective (by sending team members out, you can find interesting backstory tidbits about your recruits) and it provided nice, quick bursts of gameplay for when I didn't want to move the main story forward. Even the weapon system was fine by me, though overall I do prefer to blacksmith, and the Mark of the Stars don't replace runes in my heart, but they essentially work the same way...except with less customization (since every character has a pre-determined set of skills). Where the gameplay really falls apart is the battle system and the dungeon design. The dungeons, plain and simple, were boring and more of a chore to complete than anything. Aside from a few, they were alll just long, winding corridors with lots of dead ends. Add to that a high encounter rate and I don't think I've run from more battles in any game. Suikoden games have always been on the easy side, but this one was just way too much of a cakewalk. When not running from battles, I was just auto-battling it through the game...even on many of the bosses. I think I killed the majority of bosses in 2-3 rounds max (including the final) and I'm pretty sure I wasn't overleveled. Even with my complaints, I had a lot of fun with the game as a whole so I don't think these aspects ruin the game by any means.
Graphics: It's a great looking game, esp. considering it's on the DS. The 3d character models look like ass, but the background art and the settings are great. Also, I thought the character designs were all really good. I always look forward to seeing how all the 108 look and I thought the designs for this cast were on par with the best.
Anyway, not really a proper review there but just some random thoughts. It's not Suikoden IV by any means, but Tierkreis IS a Suikoden game and it felt like one to me. So overall I was satisfied and enjoyed the experience a lot. If I had to rate it, I'd probably give it an 8/10 . For reference, I'd rank the games like this:
II>III>V>I>/=Tierkreis>Tactics>>>IV
