A data access layer usually consumes 30% of the development time. A reasonable ORM can save you 90% of that 30% time. One such big hit is NHibernate.
Steps to use NHibernate:

• Decide - Model first, Database first, or Code first
• Create Mapping
• Configuration
• Query!

Pros:

• Caching
• Detached object
• Concurrency
• Interceptors
• Batch processing
• Attributes
• Fluent Syntax
• Database independency
• Pretty mature, and large community
• Open source
• Model classes are POCO

Cons:

• No visual modeler
• Alot of mapping code that requires to be put into
• Classes must have a no-arg constructor
• Classes should have a  private Long id;
• Classes may have private database attributes
• Clumsy mapping/configuration files

Note that, to handle mappings/configurations, Fluent NHibernate is there. Fluent NHibernate is not a replacement for NHibernate or LINQ but merely a helper library that helps you configure NHibernate in code rather than using XML files; Fluent NHibernate defines helper classes which significantly facilitate the task of providing the correct configuration.

• No good LINQ to NHB! still under development – very, very young!
• Stored procedures in NHB; challenging.
• Increased startup time due to metadata preparation ( not good for desktop like apps)
  
• Huge learning curve, especially for those having no ORM background.
  
• Pretty challenging to fine-tune the generated sql. For instance, see following SQL:

   1: SELECT    this_.[CHEQUE_ID] as CHEQUE1_0_1_, this_.[CHEQUE_TYPE_ID] as CHEQUE2_0_1_, this_.[BATCH_ID] as BATCH3_0_1_, this_.[ENTRY_DATETIME] as ENTRY4_0_1_, this_.[STATE_ID] as STATE5_0_1_, this_.[MICR_AMOUNT] as MICR6_0_1_, 

   2:         this_.[CHEQUE_NUMBER] as CHEQUE7_0_1_, this_.[PHEONIX_REFERENCE_NUMBER] as PHEONIX8_0_1_, this_.[ACCOUNT_NUMBER] as ACCOUNT9_0_1_,this_.[ACCOUNT_TITLE] as ACCOUNT10_0_1_, this_.[BRANCH_CODE] as BRANCH11_0_1_, this_.[ENTITY_ID] as ENTITY12_0_1_, this_.[FRONT_IMAGE] as FRONT13_0_1_, this_.[BACK_IMAGE] as BACK14_0_1_, this_.[IS_LINKED] as IS15_0_1_, this_.[CHEQUE_AMOUNT] as CHEQUE16_0_1_, 

   3:         this_.[LINKED_CHEQUE_NUMBER] as LINKED17_0_1_, this_.[LINKED_ACCOUNT_NUMBER] as LINKED18_0_1_, this_.[LINKED_CHEQUE_AMOUNT] as LINKED19_0_1_, this_.[LINKED_BENEFICIARY_ACCOUNT_NUMBER] as LINKED20_0_1_, this_.[EXCEPTION_DESCRIPTION] as EXCEPTION21_0_1_, this_.[OWNER_VERIFIER] as OWNER22_0_1_, this_.[CUSTOMER_TYPE_CODE] as CUSTOMER23_0_1_, this_.[IS_STOPPED] as IS24_0_1_, this_.[IS_OVERDRAW] as IS25_0_1_, this_.[LOCK_DATETIME] as LOCK26_0_1_, this_.[DEPOSIT_BANK] as DEPOSIT27_0_1_, this_.[AVAILABLE_BALANCE] as AVAILABLE28_0_1_, this_.[LINKED_DEPOSITED_BRANCH] as LINKED29_0_1_, this_.[LINKED_CHEQUE_NAME] as LINKED30_0_1_, b1_.[CHEQUE_ACTIVITY_ID] as CHEQUE1_1_0_

   4:         , b1_.[CHEQUE_ID] as CHEQUE2_1_0_, b1_.[ACTIVITY_TYPE_ID] as ACTIVITY3_1_0_, b1_.[Status] as Status4_1_0_, b1_.[REJECTION_REASON_ID] as REJECTION5_1_0_, b1_.[IS_OVERDRAW] as IS6_1_0_, b1_.[TRANSFER_AMOUNT] as TRANSFER7_1_0_, b1_.[Description] as Descript8_1_0_, b1_.[ACTIVITY_DATETIME] as ACTIVITY9_1_0_, b1_.[USER_ID] as USER10_1_0_, b1_.[ACCOUNT_NUMBER] as ACCOUNT11_1_0_

   5:         , b1_.[UserId] as UserId12_1_0_ FROM [CPS_CHEQUE] this_ inner join [CPS_CHEQUE_ACTIVITY] b1_ on this_.[CHEQUE_ID]=b1_.[CHEQUE_ID] 

• Hard to get session management right if used in non-typical environments ( read non webapps )
  
• Not suited for apps without a clean domain object model ( no all apps in world dont need clean domain object models) .
  
• Have to jump through hoops if you have badly designed ( legacy ) db schema.

You might find this interesting to know that, NHibernate is also marketed by the term “multi-database support”; this means, you *can* move from one engine to another (from SQL Server to Oracle); but this does not mean that you can connect to both the databases at a time. For instance, query from Oracle and save into the SQL Server; for this you will have to create another set of configuration file!

		ADO.NET	Nhibernate
Test #	Operation	Time (ms)	NH
1	100 Reads	000.609	000.859
2	100 Reads - 3 Joins	001.844	Couldnt apply 3 level join in HQL! ツ
3	100 Inserts	000.422	000.516
4	100 Updates	000.109	000.078
5	100 Reads - 1 Join	000.328	000.781

FIG1: ADO.NET vs NHibernate

Happy coding! Btw, you may also like to refer to my other articles on similar topic:

• Should I ever use Entity Framework 4.0? 

• ORM Market Analysis

• EF4: ADO.NET Entity Framework v4.0