As a long-time user of Unity:

downsides:

• Although it's amongst the best in asset management, the module does have its quirks and can ruin your day if you're not careful.
• Hard to "patch" asset/ scene files since they are binary (see 500mb comment above ). [They are switching to a text-based scene format so this will change very soon ]
• Some web-game developers complain about users leaving the game page once they see the Unity logo [ which in the free version has to be there ]. This is a side-effect of so many people having access to the free version and so much shovelware being developed. I have my doubts about it.
• Jack of all trades dilemma. It does everything but doesn't really shine in any particular category. It's not exactly a downside but surely not an upside either.
• Really bad GUI implementation. The inbuilt GUI system is a performance drain at best. [ This is bound to change with ver. 4.x ]
• Binary SDK. As a c++ enthusiast, source code access would make my day.

upsides:

• Extremely easy-to-use.  I love UDK, Corona, Cocos2D and even ShiVa but they just aren't in the same ballpark when it comes to ease-of-use.
• Awesome cross-platform support. [ with 4.0 ] Linux, Web ( plugin and flash), Windows, Mac, iOS and android. And all of them free [ basic versions anyway ].
• Well documented scripting. Some of the best documentation this side of a complex engine.
• Decent to great performance on mobile devices. Couple of tricks to make it run great but it eventually gives.
• Great community. They've got their trolls and whiners but overall the community is great.
• Asset management. Both a blessing and a curse.

corrections:
[ Daaark ] Getting the correct orientation is not a bug nor an inconvenience. It's just the coord. system they use ( common y-up system ). The problem lies rather with the authoring tools [ z-up for 3ds max  for example :| ]. Nothing some decent planning won't take care of.

As a long-time user of Unity:

downsides:

• Although it's amongst the best in asset management, the module does have its quirks and can ruin your day if you're not careful.
• Hard to "patch" asset/ scene files since they are binary (see 500mb comment above ). [They are switching to a text-based asset format so this will change very soon ]
• Some web-game developers complain about users leaving the game page once they see the Unity logo [ which in the free version has to be there ]. This is a side-effect of so many people having access to the free version and so much shovelware being developed. I have my doubts about it.
• Jack of all trades dilemma. It does everything but doesn't really shine in any particular category. It's not exactly a downside but surely not an upside either.
• Really bad GUI implementation. The inbuilt GUI system is a performance drain at best. [ This is bound to change with ver. 4.x ]
• Binary SDK. As a c++ enthusiast, source code access would make my day.

upsides:

• Extremely easy-to-use.  I love UDK, Corona, Cocos2D and even ShiVa but they just aren't in the same ballpark when it comes to ease-of-use.
• Awesome cross-platform support. [ with 4.0 ] Linux, Web ( plugin and flash), Windows, Mac, iOS and android. And all of them free [ basic versions anyway ].
• Well documented scripting. Some of the best documentation this side of a complex engine.
• Decent to great performance on mobile devices. Couple of tricks to make it run great but it eventually gives.
• Great community. They've got their trolls and whiners but overall the community is great.
• Asset management. Both a blessing and a curse.

corrections:
[ Daaark ] Getting the correct orientation is not a bug nor an inconvenience. It's just the coord. system they use ( common y-up system ). The problem lies rather with the authoring tools [ z-up for 3ds max  for example :| ]. Nothing some decent planning won't take care of.

As a long-time user of Unity:

downsides:

• Although it's amongst the best in asset management, the module does have it quirks and can ruin your day if you're not careful.
• Hard to "patch" asset/ scene files since they are binary (see 500mb comment above ). [They are switching to a text-based asset format so this will change very soon ]
• Some web-game developers complain about users leaving the game page once they see the Unity logo [ which in the free version has to be there ]. This is a side-effect of so many people having access to the free version and so much shovelware being developed. I have my doubts about it.
• Jack of all trades dilemma. It does everything but doesn't really shine in any particular category. It's not exactly a downside but surely not an upside either.
• Really bad GUI implementation. The inbuilt GUI system is a performance drain at best. [ This is bound to change with ver. 4.x ]
• Binary SDK. As a c++ enthusiast, source code access would make my day.

upsides:

• Extremely easy-to-use.  I love UDK, Corona, Cocos2D and even ShiVa but they just aren't in the same ballpark when it comes to ease-of-use.
• Awesome cross-platform support. [ with 4.0 ] Linux, Web ( plugin and flash), Windows, Mac, iOS and android. And all of them free [ basic versions anyway ].
• Well documented scripting. Some of the best documentation this side of a complex engine.
• Decent to great performance on mobile devices. Couple of tricks to make it run great but it eventually gives.
• Great community. They've got their trolls and whiners but overall the community is great.
• Asset management. Both a blessing and a curse.

corrections:
[ Daaark ] Getting the correct orientation is not a bug nor an inconvenience. It's just the coord. system they use ( common y-up system ). The problem lies rather with the authoring tools [ z-up for 3ds max  for example :| ]. Nothing some decent planning won't take care of.