Hey, not bad at all. Thought I'd give a bit of criticism since not many people here are...
The wheels are not isometric, look around, maybe google "isometric circles" or "isometric wheels" for a bit of help, at the moment the shape is strange, and not even circular. The wheel at the front is too close to the rest of the bike, and the red part above and around part of the wheel is too close to the wheel, it looks as if its stuck on, which on a real bike it is not. The wheels are undetailed, as if they have no tread, which they should have.
The handlebars should face outwards, not straight towards the person. They should be perpendicular to whoever is riding the bike. The lights at the front are not properly isometric either, and some overlap, suggesting they are actually further forward than the others, which they shouldnt be.
The metallic parts of the bikes look like plastic, whereas they should have alot more shine. I'd suggest looking into how to create shine on metallic objects. I have a tutorial to help you with that -
http://www.tcp-ip.or.jp/%7Es-iga/tip...c_objects.html
As for the habbo, he is mostly pillow shaded (lightest colour furthest inside, getting darker as it goes further out). This is a no-no in pixel art. Pick a light source, and use it.
I'm sure you appreciate my criticism. I could say a little more but I have to go right now. This is a good start =)